They're inbred.
In the first run of the show it was revealed that Peter had a black ancestor, Nate Griffin, who was a slave for the Pewtershmit family. It was later revealed that he had a family with one of the Pewtershmit women and the episode ended in a way that suggested they stayed together. One becomes an ancestor by having children, them having children and so forth. Peter comes from a line that at one point intersected that of Lois. While it is distant they are blood related. Suddenly Chris wanting to bang his mom seems slightly less screwed up by comparison.