Scientists say that having a baby with your cousin is all right, though.
OK, what if your sister is also your mother and your lover. The two of you have a daughter. Is she also your sister, and what if the two of you have a child? Is that your mom's grandchild, or her great-grandchild?
I heard about this guy who travelled back in time and killed his own grandpa, and then he figured that it couldn't have been his grandpa (because the kid wasn't erased from time), so he slept with the girl that the guy he thought was his grandpa was dating, and it turned out that thus, he literally became his own grandfather.
No wait, I'm confusing real life with Futurama again, ain't I?