Maybe she's too embarrassed to directly ask you that question. No one here can really answer that question as ... guess what... no one can read her mind.
Maybe she's trying to figure out what's your thought process and many people rely too much on sexual orientation as if it actually dictates how people think and act. The truth being sexual orientation and personality are two different things. One relies on sexual orientation as a predictor about other's motives, behaviors, etc... as its easy and lazy.
Either way, you're going to have to ask her why she asked your brother that question.