I believe that Vito would have had Democratic leanings by virtue of his being an Italian immigrant but he most likely would have been apolitical knowing how corrupt the process was.
Tom would have been more Americanized and better schooled in business administration and would have been a Republican. Probably the same for Mike.
Sonny would have nothing to do with it.... it ain't his blood.
Fredo would ask about the presidential erections.