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I have a prof that I was considering asking for an LoR, who happens to be a graduate from Cornell, with a PhD. I was wondering if this would carry any kind of weight when applying to Cornell, even though she was not a graduate from the law school itself. Or does alum status not matter, because she's not from the law school?

It's somewhat of an unknown but it certainly can't hurt. A good letter from a Cornell alum would probably be seen more positively than the same letter from a non-alum, but it's not like a mediocre letter from an alum will outweigh a good one from anyone else. Just ask whatever professor you think can speak the most highly about you.

It's irrelevant unless the professor is very well known in the academic community. It's much more important that it's a quality recommendation than it is that the prof graduated from a different school at Cornell.