Sexual Abuse Support Group

Keep loving her and support her in whatever good thing she wants. I think that by sharing the info on her step-sister she has said something that could validate protective measures - but that is for the courts to decide.
Let her know that whatever happens she has a friend in you.
Some (now adult) victims regret having been forced to talk before they were ready ... someting to keep in mind ... but all victims hate being ignored. If she talks, listen to her and keep loving her.
Some abusers will not touch the younger children, some will.

Hugs

First off, my hats off to you for being a foster parent for children like this. Children who have been abused physically and/or sexually can be a real, uh, "challenge" to care for (I know because I was such a child) and it takes a special type of person to be a foster parent to such kids.

If I understand correctly, the 10 yo is at risk of being sent back to an (alleged) abuser while that abuser is still on trial for abusing the 15 yo ? Something is slipping through the cracks; it shouldn't work that way. While of course the abuser is allowed his day in court, and is innocent until proven guilty, still it remains that if there's sufficient 'probable cause' for it to go to trial then this should also be sufficient grounds to keep the 10 yo away at least until a verdict is given.

Does your county have a CASA (Court-Appointed Special Advocate) program ? This would be a 'neutral' third party appointed by the court to look after the situation and make sure nothing slips through the cracks and that the interests of the child are represented.

Based on the history you've outlined, the 10 yo might or might not have been sexually abused. It's not uncommon for sexual abusers within a family to focus on one child while leaving the other(s) alone. In their own twisted logic, it's how they soothe their conscience; they convince themselves that because they haven't molested ALL the children, then the whole thing isn't their fault. They tell themselves that, because of this, then the ones who got it 'asked for it', and they'll try to paint themselves as victims.

As for the 10 yo, my guess is that if she hasn't spoken up by now, she's not going to until a particularly bad trigger breaks down the wall and it all rushes out.... which might be tomorrow or might be 30 years from now. These things can't be forced, even as time-critical as it is to get an answer vis-a-vis the father. Always leave the door open and be prepared, but without any expectations.

When it comes to counseling youngsters like this, sometimes Less is More. There is a risk of overload. As adults we can comprehend that sometimes we have to bite the bullet and go through what seems like an interminable process for the sake of an even greater long-term good. But children just want it to be over.

I do not have many suggestions but KUDOS to you for being a great foster parent...

I'm not religious, but if there's a God may you be blessed 100x fold.

I would highly recommend a book called,

"The boy who was raised as a dog" by Bruce Perry PHD.

Is about trauma and the brain basically, is very helpful for anyone who's in contact with children who have suffered trauma, foster parents, teachers, social workers, counselors, other psychologists........Really helpful and a great read.

Yeah i agree with portlander about CASA Guardian ad Litem, they can be very helpful if she doesn't have one yet.

OMG! Some things I shouldn't read. Triggers!

I agree. Big props to you for taking this on. Foster parents are amazing people and I have the utmost respect for them, yourself included!
In my experience, the best thing to do is to continue to support her. Let her know that if/when she ever wants to talk, you're there to listen. And then...you kinda have to leave it at that. She'll share when she's ready. As you've obviously seen, it takes time. Sometimes a LOT of it. But I hope that eventually, she'll trust you enough to open up and talk to you.
You're doing such an AMAZING job so far, so keep it up and let us know how things are going.
I'm always here if you ever want to talk, friend. You're in my prayers. BIG hug and tons of love.

*applause* good for you. Im 15 now been abused twice and I know Im difficult to dela with-half the time I cant deal with myself so kudos to you! The only way I can think of is to let her know what will happen if she tells you the truth. Make sure you do it in a gentle environment, like go for a walk or a cafe or something fun like an amusement park. She will open up more if she feels safe. Make sure she knows nothing bad will happen if she tells you ie. she wont be punished. Good luck and I hope she opens up. :)