Einstein's theory of relative titty

The more closely related to you a female is, the larger her breasts have to be for you to be enticed to get all incestual with her. As it is generally frowned upon in most cultures to bome one's sister, said sister's breasts would need to be the size of basketballs in order to tempt one into doing so. However, a fourth cousin twice removed might have non-existent breasts, but one would have no qualms about shagging her, as the two parties share only a dash of genetic material (in her vagina).