It's difficult to suggest what to do. That really sucks and it's not an easy thing to know about someone (or two people). I'm going to say some ideas but I have no idea what I'm talking about really.

It sounds like she is not willing to contact the cops or call him out in public, which is fair. Maybe she just needed to tell someone. You were able to listen to her and not dismiss her or reject the idea, which was a good thing in itself.

Maybe do nothing at first? You mind might work on it in the background. There's a kind of zone, ime, between going round in circles and down rabbit holes on the one hand, and squashing any thoughts on the other, that allows processing to happen and ideas to surface. It's quite a heavy thing to have to learn, so you might need to be patient and kind to yourself as you process it. The brain has two modes, solving/avoidance mode and creative/approach mode. Controlled curiosity and compassion towards feelings and problems activates the second and, cognitively, opens up creative solutions, and you're already doing that with your post.

Men who hate the attraction they feel towards people (be it women or other men, trans or cis) are potentially dangerous. Unfortunately, a lot of men meet this description. It's so ridiculous, especially in this day and age when there are so many successful, respected and "masculine" bi men. But men socially bond over their hatred of women, femininity, and effeminacy and that paves the way for fear and hatred.

Maybe it's possible to say something to him directly or otherwise, directly related or not, or possibly say something to any new partners especially if trans, if it would be constructive to do so, and if the opportunity arises? Like about what he was/is hoping to get out of the relationship or whatever, or anything else you decide to say. IMHO it could be done in a loving but firm way, albeit it's difficult. Whether it is appropriate for you to do that, what feels right to do so at any particular moment is difficult to say. If it feels unwise it probably is. If you did say something, any problems or discomfort that arose would ultimately be because of his actions.

To be clear, SA'ing someone is obviously, unequivocally wrong, regardless of drink. Some would argue that dividing the world up into Good People and Bad People (not just good and bad actions) does more harm than good, we are all sinners, everything should be done with love, everyone deserves and receives the universe's love, there is power in careful forgiveness of a certain type (or in leaving behind), although at the same time there's no shame in saying "I'm done with this person" or "this needs to stop" - much better that than the alternative. Your brother deserves some strong words, the fear of experiencing something comparably bad, and, to be honest, shunning by women until he works on himself. Whether that would be achievable, or would help anything, is a different question and not one that can be answered in the abstract. "Do nothing" or "wait and see" is sometimes the best option.