There may -- perhaps -- be good test cases in which you might argue that a state prohibition against incest violates the Due Process Clause. Take, for example, a hypothetical 54-year old man who sleeps with his infertile 56-year old sister, whom he has never seen until two years ago (since they were both adopted at birth), after a lengthy relationship that first began when neither knew that the two were related. That's a good case. I can see at least an argument there.
But a drunk father climbing into bed with his just-turned-18-year-old daughter who puts his finger inside her and then has sex with her as she's "quietly crying and scared" of him? That ain't gonna fly. I don't care how a legal theory might be articulated in a good test case. You ain't it. So you're going to lose.
As indeed you do. As Justice King affirms both your conviction and sentence of six years in the pokey.