I've always wondered about that expression. That would be your sister. She would be your sister, except for certain mitigating circumstances? She would be your sister, except you don't really have one; she's just a friend who likes to pretend? She would be your sister, except for that incident involving a bowl of cereal and a vacuum cleaner (but we don't talk about that)?
Anyway, you don't really care, but she's cute. And your hat is whack.