If they divorced when he was Two, and she's a freshman while he's a Junior/Senior, then wouldn't they be Half-siblings?
I mean, it’s genetically risky, but... reinforcement of bad genetic errors is the logical reason incest is bad, but the only reason it is illegal is because Lot was told "Bad!" by this "god" person in the Old Testament.
That and Leviticus are the only reasons there are laws against it.
So... since they're only half-siblings.... ¯\_(ツ)_/¯