[hocus pocus]

I'm really sorry your friend has gone through this.

I agree with what has already been said. Try to be there for her whenever she needs someone. Let her know that you're there for her whenever she would like to talk about what she went through. You don't even have to talk about her trauma to be supportive. If she looks upset at school, for instance, you could be supportive by providing a distraction or something like that. I know a lot of times, people are afraid to ask for help but just having a safe person around, whether you're talking about the trauma or not, can be helpful.

Encourage her to seek help. Like it has been said, she needs to tell someone when she feels ready and telling someone has to be her choice. But, she could benefit from encouragement to talk to someone about this. I like the idea of creating a safety plan, as well.

Encourage her to express how she's feeling. Maybe suggest blogging, journalling, artwork, or poetry. She could write a letter to her brother to express her feelings and then she can destroy it somehow. Maybe you can show her TeenHelp and suggest for her to join.