I believe I would talk to her rather than my brother, as in, "Drop it. He's my brother and I'm not going there."
If she is bi, he may already know it. Even if not, that is for them to deal with.
Unfortunately, others can sometimes spot things in relationships that those involved can't see, but if we point it out, they become upset with the one doing the pointing. That could cause even greater family issues. -EBF