First cousins are fair game in a lot of places legally, it's the social side of things that tends to be an issue. Especially here, it might be more that she's related to the main family of the clan he was kicked out of. He's not even supposed to interact with any of his family members going by what he said to his nephew. Probably applies even to random bastard children who may or may not actually be in contact with the family. They did seem like the sort that would love to find that sort of thing out and fuck with him just because they can.