There have been instances of Italians/Italian Americans marrying cousins, even in the real life mafia. Carlo Gambino married his cousin so he and his number 2 and future boss Paul Castellano were both cousins and brothers in law. Rudy Giuliani married a cousin too.
Regardless, it's such a bad idea for a movie.
Maybe
if the writing was better and if the chemistry between Vincent and Mary was there, it could work as a forbidden love kind of thing even if it's eww, but Sofia Coppola didn't help with that. Don't know why FFC was all:
https://gamefaqs.gamespot.com/a/forum/c/c3d3c4ce.jpg
Vyrulisse posted...
I enjoyed parts of 3 as well as Michael's guilt for what he became.
The scene where he confesses is phenomenal. 3 might be not great overall, but it has some bright spots.