Fh’s daughter is holding our wedding hostage

We’ve had a lot of drama with FH’s 17 year old daughter over the last 5 months. She was supposed to be one of my bridesmaids along with my 9 year old daughter and her twin brother is FH’s best man. FH talked to his kids about this before we officially asked to see how they would feel about it. We didn’t want to put them on the spot and make them feel they couldn’t say no. Daughter acted happy and excited to be in the wedding. So a few weeks later we asked them to be in the wedding and things were great. Or so we thought. About 5 months ago everything fell apart.
A little history... FH’s ex-wife died suddenly when his kids were 7. An obviously very difficult time for them to lose their mother like that. FH had them in therapy for 3 years to work through their feelings and grief and he thought they had done that as best as possible. There are still difficult moments in their lives and we recognize that. This past August was the 10th anniversary of her passing and their grandparents (ex-wife’s parents) always want the kids to go to the annual memorial service and this past year was no different. However, FH has not gone to the annual services in many years because as the kids got older he didn’t feel it was appropriate for him to be there. He always told them if they wanted him there he would go. They never said anything. Well this year apparently daughter did want him to go and was very angry with him that he didn’t go. She never asked him to go, didn’t even tell him she was going until the day before. At this point a lot of other stuff came out and she said she hated that me and my daughter moved in with them. She isn’t comfortable with her role in the wedding and she doesn’t even want to go to the wedding. She doesn’t want to be anywhere near us and she is going to kill herself.
Wow. Over several months, very expensive therapy, and many meltdowns we are now at a place where she isn’t living with us. She is living with FH’s mother in her one bedroom apartment. And she has to decide if she wants to be a part of the wedding or if she even wants to attend the wedding. She has until our RSVP deadline of Jan 29th. Then the other day I got a text from FMIL asking if we have an alterations appt for daughter and that she is on board with everything. I didn’t respond but called FH who immediately called FMIL to tell her that if this is daughter’s decision that she has to come to us and tell us. He then called daughter and told her the same thing. She acted shocked that FMIL had text me that and told FH that she thinks she knows what she’s going to do. But didn’t elaborate. Then tonight she said she needed to come over to get something. I thought maybe she was ready to discuss the wedding and her plans. Nope. Not a word. She is going to wait until the deadline just out of spite. She is basically holding our wedding hostage. We’ve already gone through the back and forth of should we cancel or not and decided that we weren’t going to let her dictate our future together. But it still is so hard. I just wish thinking about my wedding could be a happy note but instead this is what I get to worry about on a daily basis. I’m sorry this was so king and I appreciate anyone that made it to the end. I just needed to vent.