Welcome Juan -- I can relate to your sibling feud, My dad is a die-hard Red Sox fan and I'm a Yankee fan so it makes for an interesting day when they play each other esspecially when we go to Fenway for a Yankee-Red Sox game.
I've been to Fenway and Yankee stadium and I have to say the bleacher creatures at Yankee stadium are insane and can be really rude but most red soxs fans at fenway seem pretty laid back and cool with the yankee fans.