I need Help, am i just paranoid?

Ok im jealous of my gf's best friend. He is like really close to her and all and she calls him her brother. but wat made me jealous is how easily he can changer her.... She hates metal or even just rock thats not soft. But now she likes The Devil Wears Prada and then i asked her why, she said cuz her brother liked it(best friend, she dont have any real brothers). Also she says she cant forgive him for wat he did to her sis(something real bad) yet they are still best friends. Wat does this all mean?