my best friend would have to be my girlfriends sister, there's things i can ask/tell her that i wouldn't dare tell anyone else not even family, she's always there when i drop a bollock and she'll tell me straight if i'm being a ass wether i want to hear it or not and vice versa. my girlfiend knows about it and is cool about it
That's the way my sis and husband are. It's funny, I've spent most of my life watching guys I like fall for my sister. But her relationship with my husband isn't threatening.