Nature says incest is good if you can't get it anywhere else.
Saying "ewww" could mean the end of the species if it came down to the crunch.
I think if humans became endangered something would click over in our brains and relatives would start looking pretty fine. The only reason it grosses us out is because ideally an organism breeds with the goal of genetic diversity, if the option is there it is preferrable, if not then lousy offspring is better than no offspring and I think our instincts would adapt to make the best of a bad situation. It happens to animals all the time, despite popular opinion they aren't any more mindless fucking machines than we are, they aren't attracted to their sisters either usually, but that changes when they are the only 2 of their species on an island. I see no reason this wouldn't go for humans as well.