Ignoring uncles, cousins, siblings, etc., what makes you a "legacy" to a certain college? I'll put this in context of myself, because keeping it hypothetical is too much work.
My father got his bachelor's from Cornell, and his master's from Boston U. However, despite the fact that the degree reads BU, he did most of his studying at NYU. My mom got her bachelor's from the University of Vermont, her law degree from Duke, and her MBA from NYU. My other (step) dad didn't complete college, but went for a few semesters at a UC (I don't remember which one). My father's girlfriend (very long-term) got her bachelor's from Boston College and her MBA from NYU.
I'm obviously a legacy from Cornell and UVM, but what about Duke or BU? Am I a legacy from NYU, and if so, whose degree counted for that? Am I actually a legacy from Boston College or that one UC?
Also, if my father with his bachelor's from Cornell didn't give that much money, but my mom with her law degree from Duke always gave whatever donation was asked for, whose legacy status would likely count more with the admissions people?