My(f21) fiancé’s(m22) parents are making his sister rescind her coming-out during church service

TL;DR: My fiancé’s parents are making his sister receive prayer/apologize during service on 9/3 for recently coming out as gay, and we’re looking for advice on how to help her

The reason I’m making this is because my fiancé’s and I need help with a situation on 9/3 that recently came to our attention. My fiancé’s parents are extremely religious, but they’ve been worse to his sister (fifteen) than him. When he was growing up, they didn’t force him to attend like her, and he admits that he was a golden child. He would miss church/youth because of sports, and their relationship was fine when he moved out because they weren’t as strict on him. He was allowed to work and have secular friends, and they didn’t make him do purity like she did. But we were recently made aware of something that will make us rescind their invitation before cutting them out of our lives, but we want to ask for advice before doing that

His dad made a Facebook rant about how he discovered his sister being gay due to parental controls showing search history, and he also ranted about the importance of them as such. He also said he spoke to someone at church and that she probably will too. However, on 9/3, she’s going to be prayed over during service, and we’re worried about her/how it seems she’s being made to apologize, and we want to ask if there’s anything we can do (the church is fairly big with over a thousand). We’re going to uninvite anyone who attends that Sunday in agreement with her parents, but the wedding isn’t our priority right now. Is there anything we can do to help her out of this situation while we still have a connection with her parents? We’d especially appreciate to hear from anyone who might’ve been in similar situations with unacceptable parents

edit: *we live in New York, and the church where his sister is being forced to receive prayer at/apologize is on Long Island, NY*

*We’re trying to see if we can see her sometime next week before 9/3 as school is out, and we’re on good terms with her parents as of now. We live in New York too, but a few hours away from them*

*We want to talk to her in-person due to the parental controls on her advice that started this whole mess when her father outed her on Facebook, and we’ve taken her shopping in the past without her parents, so that’s the angle we’re thinking of if we can see her*

*We’re debating what to do if we see her and she says she doesn’t want to be present on that Sunday and if that would be enough to have her stay with us through Sunday without legal issues, and we know that that would burn a bridge with her parents too. Her safety/mental health is more important, but we’re considering if doing that would hurt our chances of potentially taking her in before she turns 18 if it comes down to the courts*