Liz,
Usually, one is considered a legacy if their grandmother, mother, or sister was an initiated member of XYZ sorority.
Aunts, nieces, and cousins sometimes don't count.
Some sororities (including mine-AGD) consider step-relatives while others don't.
Maybe this thread in Greek Life can help:
http://www.greekchat.com/forums/ubb/...ML/001451.html
To find out about who is considered a legacy, check the national websites of the sororities...they may have answers for you there too.