I know this sounds repetitive, but just be aware of what you might be getting yourself into.
In this situation, though, your putting your friend in a really awkward position.
Who is she supposed to side with: her BFF or her brother?
Are you willing to possibly lose your best friend to take a chance with this guy?
And if she says yes, then decide whether or not you want to go for it.
You might get to know someone in a class one semester and then never see them again.