If they even were to invite you, then take that as a wake up call that you are doing something wrong. You should have disowned them - and they should know without any doubt whatsoever that you've disowned them, probably a long time ago. No way should they even think about inviting you.

If you want to keep peace in the family, then break all ties and do not associate with them in any way. Pray for them certainly, keep praying for them, but stay away and keep the rest of your family away from them at all costs. They are too far gone for any "dialogue" whatsoever at this point and can only bring disaster to peace in your family.  

No, it is not okay.   Not even the Catholic Answers crowd would tell you it's okay.  

The "keeping peace in the family" is a rationalization - thinking up a good excuse to do a bad thing.  It's also known as human respect.  Pontius Pilate suffered from it.

I feel like this is the "keeping peace within the family" rationale taken to the extreme. I'll admit I have in the past used that rationale to defend people going to their family's Protestant marriages and what have you, and now I'm starting to see that perhaps I was wrong, but there's still a key difference here. A marriage between a Protestant couple may still be a sacramental marriage. A marriage between a man and a woman within another religion is still a valid natural marriage. It still obeys natural law. Such a couple would not be guilty of the sin of fornication, for example. A ɧoɱosɛҳųαƖ marriage is an abomination and a sin, defying the very nature of marriage itself. One may argue that a non-Catholic couple's wedding is just a celebration of their marriage and not their religion, but for a ɧoɱosɛҳųαƖ marriage - well no marriage actually takes place. A mortally sinful mockery of marriage does. Attending such a "wedding" is willfully viewing a blasphemy and a vow to mortally sin.

Do you think St Paul's words in 1 Corinthians 5 are relevant?  Does it matter if they don't profess to be Christian or Catholic?

To be clear, I speak of associating with them, not attending their celebration of wickedness, which is *of course* illegitimate.

Our hero, St. Paul, is speaking of incestuous as well as hetro fornicators - that we are to have nothing whatsoever to do with them. He is saying that it matters not whether they profess the true faith or not if you know they are fornicators, have nothing to do with them.....  [11] But now I have written to you, not to keep company: [even] if any man that is called a brother be a fornicator, or covetous, or a server of idols, or a railer, or a drunkard, or an extortioner; with such a one not so much as to eat".

From the Haydock Bible:
"...But you must take notice, that I mean, when they are brethren, or Christians, not when they are infidels, for this cannot be avoided, especially by those who are to labour to convert them..."

I say no.  Even most Novus Ordo websites confirm my no.

If one of you had a cousin or niece who was getting married to a member of the same sex would you attend the reception, just to keep the peace in the family?

I say no.  Even most Novus Ordo websites confirm my no.

If one of you had a cousin or niece who was getting married to a member of the same sex would you attend the reception, just to keep the peace in the family?