First off...thank you for attempting to do a search.
Legacy definition varies from sorority to sorority. It almost always includes mom, grandmom, and sister. Cousins in many sororities are considered "courtesies". Alpha Sigma Alpha has recently expanded our definition to include stepmoms, stepgrandmoms, stepsisters and stepaunts. Most sororities have rules about how many rounds a legacy must be invited to attend, and procedure for how to release (cut) a legacy. In some cases, sororities must call the relative and explain to them why a legacy was cut before doing so. I believe all sororities have the rule, that if a legacy attends the last round of rush (pref), they must be on the first bid list. Courtesies mean that PNMs are given special consideration, but legacy rules do not strictly apply.
Basic quota definition is number of girls rushing divided by the number of sororities. 100 girls rushing, 10 sororities, quota = 10 (easy #'s for example's sake). It gets more complicated then that, but that's the general idea.