Spoiler alert! Something we were not told in the film
I think maybe the incest goes way back to the grandfather. The Yale father implied his brother was passing on the"incest gene" to his daughter (no such thing) but the same gene would have gone to himself so the father thinks the whole family is cursed. Did the son actually graduate from Yale? The father seemed to think his son was a failure in not getting his PHD and becoming a published tenured professor. The son lived above his business, a small restaurant in an isolated suburb of where..."Boston"? Living above your business is a "Shtetlekh" thing of back in the old country or early days of immigration in the ghetto.
I thing the conflict was mainly class oriented. First the son/chef Davis is having trouble with his girlfriend.Her appearance as coldly pretty but not alluring suggests class snobbery. She is a self conscious "artiste" a ballet dancer.Her interest in him rests on can he support her financially and socially in the social set she thinks she is entitled too.He is suffocated by the family, girlfriend and societal expectations of what he "should" be working at and the part he should play as a successful intellectual instead of preferring to work with his hands and also form a business.
So Davis jumps at the chance for another life immediately attracted to his cousin which at first he does not realize he is related.His cousin Alexis is obviously unstable from the very beginning when after waving at him from across the restaurant runs in panic when he approaches her. Then her stalker like behavior lurking outside his apt. I think she knew the family secret. Why else take him to the shack in the woods where the projector is set up and show him their two fathers with this nude woman who we later find out is their sister...his aunt who is having an affair with his uncle. Then Alexis seduces him.
His three nubile female cousins are living on a farm with their old hippy nudist parents. They own a legal marijuana distribution store combined with a used goods store. They seem to be living a "Daisy Duke" semi-redneck lifestyle with real redneck ex boyfriends stalking them. Davis rescues his cousin Alexis by punching the redneck out. But later proves him self a coward by letting his girlfriend dominate him by forcing him to set out in detail what his feelings are toward Alexis whom he clearly is attracted to more than his GF but feels that he "should" stay with his old girlfriend because that's just the thing you are supposed to do.Rather than tell her the truth that he had sex with her he lies.Its obvious staying with he he is going to be "whipped" by her as the dominate in the family. He wants Alexis but is also scared of her, she did threaten her ex with a knife and her jealousy and revealing her love of Davis in front of the whole family incited a physical family brawl. In the end its all still unresolved when he senses she is still out there stalking him when he hears what he thinks is her truck pull up and shut off motor. In the end he is outside looking around fearfully and at the same time hopefully that she is still out there...will she kill him or love him....we will never know.
PS First cousin marriages are legal in New York State and Mass which no restrictions.Cousin couples have only a slightly higher incidence of birth defects than non-related couples. 26 states allow first cousin marriages