SO's & Friends/Family

Anyone ever had problems with their signifigant others and their friends or family not getting along?  Basically, everyone likes my new girlfriend, except my sister, who inexplicably hates her with a passion.  I've asked my sister why, and she jsut answers to the effect of "I jsut don't like her, that's all."  My girlfriend really doesn't have anything against my sister, so I don't see why there's so much animosity the other way.  They're both pretty important people to me, and I'd like them to at least get along.  Any ideas?

uhm scratch that last post. take them to a restaurant individually (Subway preferably) and find out what they have in common. then ask them both to come to the mall with you and take them to the store that has the stuff they both like.