All right darlin'...you're an alumna...because you're a woman. And a bunch of us are alumnae.
Men - one is an alumnus and more than one are alumni.
And your college's organized group of former students is referred to as alumni, because it has both male and female members.
Informally, I'm an alum and a bunch of us are alums.
One time there was someone posting here on GC that claimed to be a legacy of something like 9 different NPC groups, or it might have been one of those “weird rush stories” where a PNM said it during recruitment. It’s not a very probable scenario (it seemed doubtful in that particular situation, whatever it was), but definitely possible if you have a large enough family where most women have gone Greek. Of course each group has its own legacy policy, so you’d have to make sure your grandmothers joined sororities that consider a granddaughter a legacy.