This is assuming I get to be the little girl, I suppose.
Sei is awesome, but she seems more keen on abusing other people's sister rather than her own, so not her. Eriko is straight, Rei is too manly, Yoshino is crazy, Shimako is so passive I'd rather have her as a little sister.
When you think about it, the red rose family is where it's at. Sachiko has too many issues to deal with though, so I guess Yumi or Youko would be best.