While reading about the miraculous medal is promising, if the family member lives outside the home there isn't even a way to put the medal hidden in the bed between the mattress and the box spring. 

I tried praying several different novenas, but it didn't work. 

I'm also more in a hurry to convert because the family member is on opposite-sex hormones to become a trans person for about 3 months now. If she stops now she might be able to go back without significant changes but the longer she is on the hormones, the more difficult it would be to come back. All those facial hair growth would have to be removed with expensive lasers, and the deeper the voice becomes, the harder it is to bring back the voice to normal range, etc.
Also a thought occurred to me that in the miraculous stories of conversion using the miraculous medal or some other method such as intercession of a certain saint, only successful stories are mentioned, and it's not known what the success ratio is compared to those who used these means but didn't obtain the wanted conversion. 

Prayer is not an exact science, like engineering, for instance.

You cannot do X and be sure that Y will result.

That said, If I were in your situation, I would offer some sacrifices like fasts and mortifications for this cause, plus some more daily Rosaries.

It might not save this person from becoming a strange creature, but it might bring God's mercy upon this soul. There are last minute conversions, but they are not very frequent. This might be one of these cases in the future, who knows?

You can't put a time limit or deadline on your prayers.  If it will be God's Will AND the other person chooses to respond to Hi Will, it will happen.  

I was encouraged by a saying that St. Joseph's intercession has the force of command with Jesus and Mary because he was the head of the Holy Family. So I prayed a novena to him but it didn't work. So I've been just reciting a short prayer to St. Joseph most days after that.

I'm also praying 5 decade rosary every day. There also prayed 15 decades a day at one point but then reduced it to 5 decades.

I also started praying the Memorare, which has the words "never was it known that anyone who fled to your protection,
implored your help, or sought your intercession,
was left unaided"

However when I reflect on the words "never was it known... left unaided (or "rejected" in the original Latin), I feel like there have been many times in history where one prayed for something but did not obtain it. The words itself sound like a guarantee, but maybe it's just a strong expression of insisting to Our Lady to grant the prayer.

Forgot to click the button.

If you are sincere in your prayers God will surely answer them, but maybe not in the way you want them to be answered. Remember, He will always do things in the most perfect way possible.

My point is that it's not that the prayers "aren't working", but it's either that your dispositions are not good or God is answering your prayers in the best way possible which is not what you want.

Prayers, especially the Rosary. And don't foget the Green Scapular!

You can't control your family members, however good intentioned and objectively good your feelings/plans are.

In the end, each person has Free Will and will answer to God for their own choices and sins. You can't control them, nor save them, nor are you that level of responsible for them.

Not even parents have full control over their children (but keep in mind that parents control and influence more than they think, and most are, on average, probably MORE guilty than they think they are -- but I digress).

God isn't going to violate the Free Will of one of His creatures. God doesn't take back His gifts.

Also, you shouldn't look at your prayers like "working" and "non-working". Stick to regular praying/fasting/sacrificing, ALWAYS remember that "God's will be done", and don't get into "never been known to fail" territory. That slides into superstition (transactional religion) very quickly. Perhaps for some people more than others.

You don't "wield" God like someone wields one of the Infinity Stones or some such nonsense. That's not how God operates. You don't control God. Going after such "power" quickly leads to the Dark Side -- seriously though, it does lead to satanism, who is the only one who promises such "power" over creation.

Great advice.  If anyone received a superabundance of graces, it was Judas, who spent nearly 3 years in close proximity with Our Lord Himself ... and yet he too, in the end, exercised his free will to reject them all.

In terms of "wielding" God (others have likened using God as a vending machine), I've always remembered the story told by an early Church Father.  So, there was a man fishing on a lake, and at one point he catches something, which must have been very substantial due to the resistance on the line.  So he pulls with all his might for a long time, and finally sees his catch drawing closer, eventually realizing that his line was caught on a large, immovable boulder, and while he thought he was reeling it in closer to the boat, the entire time he was actually pulling the boat toward the boulder.  That is what prayer is like.  We don't "reel in" God, but merely draw ourselves closer to Him.  Even if OP were to pray, fast, do penance hard enough to "win" the conversion of this soul, it was God Who inspired him and gave him the graces to do it in the first place, i.e. it's always God and only God who converts.  God desires the conversion of this individual more than OP does, and more than the individual does, infinitely more.  This doesn't mean to be complacement, so you should do everything you can, but always realizing that it's God insipiring you and giving you the grace to do it.  God's economy of grace can also be likened to young children who have no way to earn money, can't get jobs, etc.  But the parents give the children an allowance.  These children then save their money for a long time and buy the parents a gift.  Ultimately, the parents were the source of the money, and in a way financed this gift, but what is moving about the gift is the generosity of the children in saving the money for their parents rather than spending it on themselves.  Any graces that may funnel through us to others originate with God in the first place.

I would like to ask anyone who sees this thread to pray a memorare for her detransition and conversion if you are ok with that.

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Has anyone successfully prayed for someone's conversion? What was the method of prayer used?

Would it be effective to ask a novus ordo Carthusian monastery to pray for her? 
They have only one meal per day.