The Midge doll was part of a family set (she had a husband and son, and they both wear rings) and, I must confess, I bought her. ONLY because I was desperate for Mattel to make something like that when I was a kid, because I had to make Barbie look pregnant without the benefit of a think tank (I taped those little Barbie bike helmets to their stomachs, and it always looked funny).
As for Barbie's "sisters": I think that the two youngest ones are really her kids. I mean, wouldn't her parents be WAY Too old to have any more children?? She just says they're her sisters so people won't think she's a slut.
http://community.livejournal.com/ltd_brands_suck/