Oh! there is incest action!!! Sorry if I accidentally misled you (I thought you were asking if we could "romance" them)... You can fuck Gran and Sis... you just don't "Woo" them, both cases involve taking advantage of sleepy time. So far you can "woo" Rose, Hannah, Dorothy, Vanessa and Bella.Thank you very much. Hopefully we some incest action in the next patch...although technically speaking, when you the questlines for Dorothy, Vanessa, Hannah(?) aren't they related in someway.
And Yeah, I do believe Dorothy is Gran's Sister, so her and her daughter are Family too. Vanessa and Hannah are friends of the family IIRC... though in this town they are probably related too (cousins probably). And you can fuck Mom too in the Side content.