Talk to your family members about your concerns. Ask about why they need to hide from other views if their views are actually correct, wouldn’t they be able to easily refute any nonsense argument that might be made against their faith? Come up with ways to make sure that any family member who wants to can contact you. You may want to give have them write your phone number in a Bible, or mark the number verses in their bible so that they can always call you in an emergency

I would suggest to give it a little more time, as some others have, and back away, as sometimes trying to show the "wrong" in a situation makes a teen want to defend it as much as they would the "rebel" in a movie from the 1960's. She may make a friend to take her interest away from the cult, or to get her to question why she is going along with them. It could be from outside the group, which would be ideal, but it would need to be something she chooses. You couldn't introduce her to it. All I can say is good luck.

If it where my son in law indirectly accusing my other daughter of being some kind of child molesting pervert for wanting to care for her nephew, he and I would be having some serious words in private, a conversation including such words as, Fucking, idiot, pervert and sledge hammer"
This is nothing to do with religion, this is a seriously twisted individual who need putting straight now, before this goes any further.

You can not, nor should not, force another to adhere to what YOU think is right. One can only present their way of thinking and reasoning and hope it will have some influence on another.

My first instinct was how disgusting but... So the husband and preacher are the same? or both are creating this influence on the family? and the husband insists on being the only one to exclusively change, bathe and dress him? Praise the lord and pass the pampers. Seriously my sympathies, just trying to look at the bright side ?‍ If you really want to help huck whitewash the fence she could turn it around on him and insist that she be the only one to do it because the sight of baby penis is too great a temptation for homosexuality. ?

I would say that if there weren't children involved, this would be a case of grown ass adults making grown ass decisions regarding their own personal lives. The fact that there are children involved changes the whole equation.

Keep an eye out for signs of neglect and abuse in the kids. At the first sign, bring social services down on these pricks like flies descending on shit.

You can't. Yes, they sound wack, but they are adults and are entitled. Make no comments. Your home should be a shelter for the one grandchild if and when she needs you. Step carefully but don't back down. I feel for you.

I thought there were organizations that help with this kind of thing.

I think this presents multiple hurdles. First, to acknowledge it's unacceptable and second, the courage to leave or put her foot down.

Educate them on how cults work. Try films or books from escapees.

So sorry. That is a hard situation. It's not abusive is It? Another person posted that these are the same things that happen in abusive relationships. From the little information it sounds like he is very controlling and is trying to limit her relationship with her family. Make sure she knows that you still love and support her regardless. That still doesn't help though because it is hurtful and you obviously want to see your grandchild. ?

Exact same things happen in Abusive Relationships. I think those with Narcissistic Tendencies often gravitate to religion as a means to validate the abuse or affirm their superiority to everyone else.

I would point out that no one is fanatical about something that is not in doubt. (try shouting at her that the sun will rise tomorrow to illustrate). If they are so afraid of outside influences. What have they got to hide?
Promise not to talk about god or anything that will cause an argument,. Just keep it family