Of course you are concerned! Of course you are at a loss. No parent is prepared for something like this. I’m so glad you wrote.
First: Some secrets shouldn’t be kept. You are right that both kids are troubled. Your daughter may feel safer now that she is sleeping in your room, but that doesn’t make the impact of the situation go away. My guess is that she is upset with herself for not telling you right away and upset with her brother for violating her trust.
Your daughter needs to be told that pretending to sleep when she was being assaulted is a normal response. She didn’t know what to do. Why would she? So she just froze. It’s what kids in such a situation often do. That is not something to feel guilty about.
She shouldn’t feel guilty about telling you either. The most caring thing she could do for the entire family was to tell. Please — tell her that you are going to talk with her brother and get to the bottom of what this is about. Denial of the situation won’t guarantee that it will stop nor will it get him the help he needs.
Then talk to your son. Don’t put it off. Approach it with as much compassion as you can muster despite your justifiable anger. He already knows that what he did was unacceptable. Unless he is mentally ill, he feels guilty about it. He knows he needs help. One thing to ask him is if someone has sexually hurt him. Often kids who abuse other kids are acting out something that happened to them.
Ever since you moved her to your room, everyone in the family must know that something is up. It won’t surprise either of the kids that you looked for advice here or that you will insist that everyone get into some therapy.
Find a therapist who specializes in adolescents and sexual abuse. The therapist will do an evaluation and will give you guidance about what to do next. Please don’t try to go it alone. You need support as well as practical help so that you can help both of your kids — and deal constructively with your own shock and distress about the situation.
I wish you well.