If they are otherwise free to marry, and are simply cohabiting, if they do validly marry, at least that will put an end to their near occasion of mortal sin, viz. repeatedly fornicating.

I can't see a problem with congratulating them on their upcoming wedding.

 Dear Boomerang: I don't like Simplemans advice.: Cohabitating, just marry.  Oh, where is the sorrow, confession.  My mother did this.  My dad died and 5 years later she takes on a divorced protestant.  Mercy, I regret having said in 1987, congratulations.  So, in 2000, I had a short talk with her.  Come to find out ,the not so- priest worked a deal out. (rigged) You marry this man out side the church, then file for an annulment(from his divorce situation) Then my mom could have no sacraments (being she married outside the church as the priest directed her to do). After the annulment went through at the cost of $1,000 then my mom was told to go to confession and all would be in order. Ha! What a lie.  

 I said,Oh, mom, he lied to you.  You excommunicated yourself, and you were lied to, to make things look right.  Well, I told my mom I could not congratulate her after 15 years, that it was a bad influence on the whole family and others around her, scandal. My mom did not like what she heard from me, neither did my siblings of 8.

I was disliked but not exactly kicked out.  But I can't begin to say how much damage it did and continues to even after her death.

Never be afraid to say what Christ would have said, "It is against the laws of God".  Vatican I needs a good reading if you can read online or books.  After 300 years of war against the Church it was boiled down or rooted to 1. Against the Incarnation of God (we don't believe in a savior) 2.  Lawlessness, ( we want no authority, don't tell us what to do)

We don't play the gave of False peace, even in saying nothing.  Say it once, where your heart is, and they will know and when you die you can meet your maker and say I spoke up for Thee!

I actually thought of this, but seeing as they are planning to marry, and marry validly, it would be incuмbent on the priest to tell them that they need to go to confession before receiving the sacrament of matrimony.  If they are resolved to continue in cohabitation, they might put off that confession until a few days before the wedding, but better that, than not at all.

I'd much rather see a cohabiting couple marry than not marry.

Thank you for the follow-up, in the cases I'm dealing with its Catholics marrying an unbaptised person in the Novus Ordo/Indult, or just unbaptised family members getting engaged (so a natural marriage (?), but with the nowadays cohabitating et al beforehand).

There is this post I have found for the Catholic cases
https://www.cathinfo.com/catholic-living-in-the-modern-world/are-people-married-in-non-catholic-ceremonies-really-married/msg859528/#msg859528

but given the confusion of Novus Ordo having "jurisdiction" could the Catholic really be dispensed to marry an unbaptised person?

Sound, Catholic advice:

Thank you, I'll listen to it soon

Should you rebuke people who do not care about religion? What if they are relatives? Or non relatives but extended family from in laws side? Eg brother in laws brother.

There is no tidy way to deal with this as we live now in a post christian society and these people are reflection of the world today. 

But you don't have to have them stay in your home, nor need you visit them.

When it comes to direct family, after 10 years, I'm still smarting from the rebuke of my father when I informed him that I didn't want his new 'wife' to send presents to our children. In his response by email he copied in every member of my family to show how judgmental I was. I therefore do not recommend writing a thoughtful, kind and well explained letter. I had agreed with my sister not to congratulate him, but she went ahead and did so, so I was very much on my own. 

We still talk, and I have met up with him and his 'wife', but never at home, so my childrens' only experience of their grandfather is unfortunately rare meetings in restaurants. 

I can only say, "it's not easy" to cut yourself off from family and relatives for the faith. Deo Gratias my parents did that and ended nearly all contact with all my relatives by the time I was 10 years old or so because they all went gung ho NO.

 I have siblings who lost the faith - I have had no contact whatsoever with them for the last +20 years. On one hand, it is not the greatest thing in the world, otoh for me at least, it's best. At least it's one less obstacle to deal with. Should any of them ever change their mind, they know where to find me. 

As for friends, I've had work acquaintances but never had any friends outside of work, and the friends I had and have, were always from Church or we met one way or another through the faith. 
 

Except for my parents and one aunt and uncle, my relatives went gang ho into the world. It happened gradually over the the first dozen years of my childhood. People moved away after careers and the like. Close contact was lost and when contact was made, it was evident nobody was practicing the Faith except an aunt and uncle. As my cousins hit their mid to late teens, they were no longer attending Mass if they ever had in the first place. Several cousins married outside the Church, one joined the rainbow brigade, all of them divorced and are shacked up, or dating because she’s “single again”——in her late 70’s. An entire branch of my family disassociated themselves when I was age nine, in a dispute over a will. My parents refused to take a side because it was pure greed and actually ridiculous. Refusal to involve themselves resulted in all of us being cut off. In reality, they cut themselves off. Nobody knows where they, or by now, their grandchildren and great grandchildren are at. Of the entire lot, they were the most Catholic. Such irony!  Of those who remained Catholic, I’m the only one still living. One first cousin showed up at my father’s wake. I didn’t recognize her. 
As for friends, I basically have one left who I met at Mass while I was car camping on vacation. It turned out several of her children basically grew up with the boys of a family, their parents among the pioneers of tradition. Several of the boys became priests, one of whom was instrumental in my finding out about tradition. So there was an instant connection. We live 1,000 miles away from one another, but we do talk on the phone regularly. My other Catholic friends have all died in the last seven years. I used to have some Protestant friends, mainly from work connections, but we were never what one would call super close. Once they moved, retired, passed away, that was it. I do have a sister and two nephews living nearby, but we were never on the same page. She also married outside the Church. They had two boys and they split up, acrimoniously divorced. The boys have little to do with their father. My sister’s religious education was even worse than mine. I grew up knowing God existed, that Jesus was His Son and Mary His Mother. I went to Mass, novus ordo, and came out believing in God, Jesus, Mary, that there were saints and angels, that I could pray to them. I learned that in about four months of weekly catechism classes with a real nun, Sr. Mary Veronica. My sister came out believing there was a God in the sense of a higher power. I’ve tried, of course, to talk with her, but now confine myself to prayer. 

I converted and officially came into the church in 2013. This has been my “bitter pill” ever since.
There is no way around it.  It’s going to alienate you from relationships with friends and family and it’s going to hurt. I’m sorry.

In my years of dealing with this, I have found that there are some absolutes and there are some situations that you simply handle with prayer and see how they go.

As far as weddings go, if I know that they will be marrying invalidly, as in one or both are “divorced” from a valid marriage, I will not go. If it’s not a close family member, you don’t have to say why, just say you can’t attend. If it’s someone like a sibling, it’s just going to have to be a hard conversation. You’ll probably need to tell them why you won’t be attending and be prepared for things to change in that relationship from then on out.

With co-habitating I’ve handled it in different ways, I’m not sure there’s a cut and dry answer, I think it depends on the situation. For example, my husbands best friend from childhood wants to visit us and stay for a few days. He is living with his girlfriend. It’s already been known that if he is bringing her, they will be provided with seperate sleeping arrangements so as not to set a bad example for our children. It didn’t cause a problem.

Many NO priests will answer these questions with extreme laxity, claiming that if it will cause big problems or a rift in the family that attending an invalid wedding is permissable. I would say they are wrong.
Our Lord tells us to deny ourselves (in this case, our “need” for human respect), pick up our cross and follow him. He also tells us that when we follow him that the Holy Ghost comes to comfort us.
To follow Christ is to suffer. It can be so lonely. I’ve been lonely for friendship since becoming Catholic. I lost all of my friends but one and the relationship with my family changed forever. Neither my or my husbands family is Catholic, and to get to a good Mass we have to travel a hard distance. There is nobody in our rural community with whom we can share in commonality of the Catholic faith. The scorn that inevitably will come from it is hard to take. We can always turn back, but we would lose Him. Don’t turn back!
Do the hard things, but also don’t be needlessly austere. Pray for wisdom in every situation and take comfort in the thought of eternity in our Savior's presence! When we keep our eyes on him, the road doesn’t seem quite as long and hard.

We have not “cut out” anyone, but we have set firm boundaries and explained our position when asked. When they realize how religious we are and are going to stay, it is they who do the leaving.

Well said!

When my father’s cohabiting friend (Jєωιѕн) while still married to his legal wife,(also Jєωιѕн) wanted to stay overnight at their home while on a long distance bicycling tour, he was in a quandary about how to gracefully say no. He and his legal wife had stayed at their home before they broke up, so it made for a dicey situation. Neither wanted to deal with his volatile temper or his shack-up honey because she had lured him in on another bicycle tour. My parents both knew her, plus, she’d made a most inappropriate pass at my father at a bike club meeting. My mother said, “I’ll be glad to tell both of them off and break the news to Jay that his concubine had only, several months before, propositioned my husband. Over my dead body will I have that woman in my home.”  
The only complication was that the man was still, in part, managing my parents’ finances. 
Maybe it was cowardly, but really, it wasn’t my parents’ place to inform Jay, and they didn’t want to place a rather large sum of money in jeopardy. So they decided it was time for a quick trip to visit grandma, uncle and aunt out of state! That’s what they did. Called the relatives, told them they’d be in the area in their RV, and we’re coming for a visit. 
Jay and Miz Other Woman found somewhere else to stay, probably tenting at the state park with the other club members. 
They had a nice short visit with the relatives, slept in their RV, went out for dinner and brunch, and my aunt ordered a deli platter for the one my folks ate in. Not that there was a religious problem, but two of mentally ill adult cousins were living there along with one cousin’s autistic six year old. His father was jail, so the home atmosphere was to put it mildly, tense. From there they took a few days to themselves touring a scenic area and stopped in to visit Dad’s old Navy buddy who lived in a veteran’s home. Arriving home two weeks later, Dad received an early email from Jay with some photos and the news that he was back with his wife after he caught his concubine “cheating” on him in a Florida campground!  
Of course, he broke off with his wife again, had two more women, one of whom he moved in with out of state. There he was killed while bike riding by a drunk 16 year old girl in a stolen car. 
I know he sounds horrible, but in most other ways, Jay was intelligent, generous, a person who’d give you the shirt off his back, literally. He used to support various charities, not just with money, but with material goods. There was a family shelter and daycare run by St. Charles Mission that he found out many people lacked winter coats and footwear. He went in person, got a list of needs, names, clothing sizes, then used his connections to order the clothes and boots from Syms a large clothing outlet. He was friendly with men of decent morals, but when it came to Commandments 6 and 9, poor guy was out of control. His own mother used to warn him that his womanizing would be “the death of you.” And she was right. She outlived him by 16 years, dying at age 103. Once I was hospitalized for emergency surgery, my rent was overdue and my folks in Ireland. Somehow, Jay found out about it and payed not only the overdue rent but a month forward. He refused to accept any payback. It’s just sad he didn’t save his soul.