So sorry that this happened to you, sarah. Your brother put you in the middle (which he was wrong to do), essentially asking you to choose between him and your best friend (which isn't fair to you at all).

Regardless of your gay friend S's actions at that party talking about the karate teacher out of context that upset your brother, the crux of the matter is that your brother chose not to seek help after his karate teacher assaulted him. He didn't tell you or anyone in your family about it but kept it to himself. Yet he expects you to fix his past, which is just not even possible.

You did everything you could, listening to your brother rant about his past. I think he picked a fight with you and your gay friend b/c on some level, he's mad at himself for not reaching out for help sooner, and so he projected that self-anger on to you and your gay friend S because it's easier to be angry at someone else, than to be angry with ourselves. I could be wrong but that's just the feeling I got from reading your post once.

I think your brother needs to see a sexual abuse therapist who has the training to give your brother the coping skills and tools to deal with what his karate teacher did to him. The therapist your brother works with, could teach him self-empowerment and teach him how to reframe his anger, which has been misdirected at your gay friend S and at you via text. https://www.counseling.org/docs/disa...e.pdf?sfvrsn=2

Do not beat yourself up about this sarah. You've been a supportive sister to your brother. There's really nothing more you could have done. He chose not to tell you about the event when it happened, possibly due to shame, embarrassment or guilt or anger or fear which are normal responses to being sexually assaulted. But he is wrong to blame your gay friend S and blame you for not being sensitive or not helping him. He's 30 years old. He needs to go see a therapist who will help him properly process that awful experience he had with his karate teacher, so that he can move forward with his life and resolve his misdirected anger at your gay friend S and at you.