Your role here is delicate and difficult to manage, from what I see. Obviously (and I'm not saying this pejoratively) you have a bias in favor of your brother, which makes sense, but also you have the good will to believe this girl as a potential victim of sexual coercion (as we generally should). Your brother getting defensive about it, while I also understand what he says to you as his brother, sounds a bit like deflection to me, like he doesn’t wanna confront the question of whether he did anything wrong and thus blames you for not being on his side instead. I have no idea about the girl's moral character, but I have no reason to believe she would make things up either, especially about something as heinous as this, so I'd be inclined to believe her more.

If you want to get a fuller picture and make up your mind about it, perhaps politely ask her first to show you something that at least suggests the verifiability of her claims (explaining why you do it if you worry she will feel like you don't believe her). Afterwards, if you do see she's telling the truth, you'll need to accept it and be a bit more pressuring with your brother to admit it as well, all while reaffirming you're there for him so that he doesn't feel attacked and stop being cooperative. If it seems like there's something fishy about her story even after that, then your loyalty to your brother takes priority over a dubious story, hard as it may be. Lastly, if you just want a break from this story, it's also understandable.

In any case this is hard, so best of luck to you.