@RobbyBevard:
She'd have to have no idea who her birth mother as in that case, since in an earlier episode she specifically had no idea who Jackie Onassis was.
I don't think any sort of deal has ever been made about her family history though. Heck, none of her history until she was at University. But making her Rusty's half-sister certainly wouldn't be beyond the show at all… any more than making Monarch a lost third Venture Brother. ...probably not both of those things however.
SPOILERS:
All the characters are related to each other. And are clones.