Need some advice

So, I’m hoping to find some advice from someone that may have some experience. Many of us in our extended family believe that my SIL’s ex husband is abusing their kids. They have 3 kids (2 girls, 6 & 8, and 1 boy, 4). I should also mention that he has a sex addiction. They’ve been divorced for about 3 years now, and he has daytime visitation (no overnights, thankfully) 3 weekends a month. Their middle child, especially, is showing a lot of signs of sexual abuse. Their therapist called CPS and reported it. They went to the school and talked to the girls and said that they’ll do something when she’s ready to talk. And they called him and told him there was a case against him, but it’s being closed. We feel that things have escalated since then. And now their 4 year old boy is showing some signs that are serious red flags as well. I guess my main question is about how to help her (the 6 year old) to feel safe enough to talk about it. She just holds it back. She looks like she has something to talk about, but she’s afraid to. She’s a very anxious and scared child. And I believe that he’s manipulating those qualities, and telling her that if she tells anyone that he’ll get in trouble and never see them again. Her demeanor completely changes on Saturday morning when she has to get ready for him to pick the kids up. And we’ve had a few talks with all of the little girls telling them that it’s never ok for a grown up to touch you inappropriately and they can always talk to any of us. But she still always holds back and won’t talk about it. Does anyone have any tips for how to talk to a child about abuse, and maybe how to help them to feel safe enough to trust you and open up about it? We just really want it to stop and get her the help she’ll need to work through it, and without getting her to talk about it, it won’t. It’s breaking my heart to see this sweet girl go through this all by herself.