When Michael marries Kay, the novel only mentions her family and friends. Why didn't come anybody from the Corleones? Not even Tom, Connie or Fredo?

Maybe they disapproved of Michael not marrying an Italian but instead a WASP?

In one of the deleted scenes from the movie, Vito asks Michael if his 'American girlfriend' got back to the city alright, and then after Michael answers and gets in the car, he kind of rolls his eyes.

I think it was a conscious effort on Michael's part to separate his wife--his symbol of impending "legitimacy"--from the family and associates who were integral to his criminal life. Also, as was pointed out, Vito disapproved. It's interesting that, in the film, we see Kay with Vito only twice--in the photo at Connie's wedding, at at his gravesite.