Very common, especially among the Sicilians. Very clannish.
My Wife's parents were first cousins.

I believe the Genovese Family liked Paul better than Neil. That could have been PART of Gambino's decision. In the old days, the Genovese's had blood relations in the Outfit and were allied with them on the Commission and in Las Vegas.

Gigante hated Gotti for two reasons from my understanding hearing about it in Chicago back in '85:
1) Gigante and Castellano got along real well.
2) Gotti raised his hand against a Boss and did it without Gigante's approval because he knew Gigante would have NEVER sanctioned it. Gigante argued that Gotti should be killed.