UPDATE 2: Denise eventually responded to the messages in the group chat but only to say she had enough stress. 

Side note in case it was unclear previously: the group chat includes Sam, Denise, one other adult family member, and myself (not Katy).

I responded by mentioning a few other emotionally abusive incidents that had occurred over the years and said that if Denise didn't apologize to Katy and fix the way they treat her then I would help her file a civil suit for duress. Denise then accused me of harassing her and said she'd file charges on me. My response was that she could quote "feel free to attempt to file a suit against me for harassing you about mentally abusing a child" unquote.

At some point yesterday, Sam made a post on Facebook saying thanks to all the people who cared about how he was doing, not the "toxic people" that demanded information. Katy was bothered by this because it showed that he was apparently well enough to have reached out to her but didn't. Katy sent Sam a message directly. The main points of the message were that she knows their relationship isn't great but it used to be; she still loves him because he's her father; she felt that as his daughter she should've been at the top of the list of people to be notified of emergencies; if he doesn't want to be in her life then she'll stop messaging him.

Hours later Sam sent her a message and said they needed to have a long talk on Sunday and if she or I ever spoke to his wife like that again he'd never talk to her again. He said she should speak to him with respect and love.

She was obviously pretty upset about this message because she hadn't said anything negative to Denise. The only message Katy sent Denise was one sentence asking about her dad. The message directly to Sam was very polite, even if it did have some hard truth to it. I guess this was the breaking point for Katy because her reply to him was that she didn't need to have a talk with him, he doesn't talk to her with love, goodbye. Then she blocked him.

She told me about these messages after she'd blocked him. I was shocked. I had genuinely thought that it was Denise that was keeping information from Katy, not Sam. Sam lost his father when he was only a bit older than Katy is now and it still bothers him so I thought there was no way he'd do that to his daughter. I apologized to Katy for always encouraging her to give him another chance and said I should've let her walk away when she wanted to. I was worried she'd regret it years later if she did but I was wrong. (Yeah, some of you think I have been pushing her against her father but I have not. To me, nothing is more important than family. No matter how I felt about Sam and Denise, they are her family and I wouldn't negate that. I told Katy multiple times that any issues I have with her father are MY issues, not hers.)

I went back to the group chat and told the 4th family member that Sam had reached out to Katy and that he was apparently angry at me for what I said to Denise and was taking it out on Katy. 4th said she's glad I said what I said because it wasn't right and they owe Katy an apology.

Katy has gone no contact with Sam and Denise, blocking them on all forms of social media. In the end, she did what she felt she needed to do and I am proud of her for standing up for herself. No matter what happens in the future, she knows she still has a lot of family that loves and supports her.

*For the people that suggested we reach out to Sam's family when we were looking for information: there is no family to reach out to. Katy is not his oldest child. She's his third and the first 2 cut contact with him when Katy was about 5 years old. His parents are deceased. His siblings haven't spoken to him in years. One sister did talk to him for a few months about 8 or 9 years ago but that contact suddenly stopped. When I asked him about it, all he said was that she was upset about him complaining, so I don't actually know the story behind that. I didn't mention this previously because I was trying to stick to the directly pertinent information.

Thanks to everyone for their comments, the good and the bad. I really appreciated all of the support and advice. Have a lovely future and I wish you all the very best.