The only reason I could think is the legacy situation. If a girl is an in-house legacy to a chapter we would probably assume she'd want the chapter that her sister is in. However, I remember one girl who was a quadruple legacy to another chapter on campus and her older sister had just graduated the year prior....and she pledged Chi-O. So I would be hesitant to judge someone based on that unless they said "just so you know, my sister is in XYZ". But then she wouldn't be cut for being a legacy....she'd, again, be cut for being a bitch.
