Dating my cousin
Maybe it was the Hillel Pharisees vs the Shammai Pharisees.I'd be more worried about the personality traits you've described than the degree of consanguinity.
Her dad died years ago, and her mom moved across the country to get away from the manipulation. If she's that manipulative, do you think that's possibly it, he's just being played? I'm presuming the conversation went something like "So I'm in love with Cousin, she's great and we're moving in together." and "OMG, eww, are you fuckin' kidding me? He's not argumentative about it or an in your face type of person, either. I know that it's not illegal - he was quick to point that out to mom when this first started.
Extended family doesn't know about it, but my parents and siblings all do. Complicating things is that my second cousin has a history of being a manipulative user.