There's supposedly something called the Westermarck effect that prevents incest from occuring; however, some studies have suggested it doesn't actually exist, and that Freud's Oedipus theories are more correct (in that family members lust after each other).
On the other hand, there's a hypothesis called "Genetic Sexual Attraction", where you're more sexually attracted to those more biologically similar to you. It's been decried as pseudoscience however.
But, if you toss "Kin Selection" and "Kin altruism" into the mix, there seems to be some truth to it; because people generally are more cooperative with and more likely to help those closer to them in family ties, building families with those similar to you genetically, may be a biological influence that helped develop society.
TL;DR there's a basis for blood and non blood related individuals to be attracted to each other in a family.
It's pretty obvious that they are going to end up together. She's the only available female who's been there since the start. :) The siblings part is weird though.