You are wrong, It`s blood, part of it, but it`s blood anyway.
I think it is so disgusting to see a person with her sister, cousin in 1st, 2nd, 3th, 4th or 1000th grade. It reminds me The Family, the Borgia family I mean. I don`t have any personal problem with people who had chosen(or not in case it naturally worked)homosexuallity, but I don`t know if being in love with relatives is the worst thing you could ever do.
Despite Michael wanted a "respectable" man for Mary, I think he did not want Vincent for Mary because they are cousins. At least if I were Michael would do the same.