Not at all. I think a good portion of girls like their BFF's brother. The thing is you can't act on it. Not unless you want to lose your BFF. Because when things don't work out, family comes first, and you will be out a BF and a BFF.
Will it be weird if you like your bff's brother?
No it's not weird. Attraction happens love happens , everything exists in the world!:)
It'd be weird for me ... Since I'm a heterosexual male.
LOL