If it's consensual (as in actually informed consent without coercion or power imbalance stuff) and there is no breeding, then I can't find any moral objection to it.

There is something called the Westermarck Effect, by which people tend to be sexually repulsed by people they have grown up with closely -- that is, their family. The evolutionary basis is pretty easy to understand. Inbreeding tends to produce unhealthy offspring. Those who don't screw their own family have healthier children with better survival and breeding odds. Evolution dislikes incest.

I think a lot of people feel the "ick" factor and then invent overreaching moral objections to incest altogether. Actually, I think this is how a lot of people develop their morals. They feel something is bad in their "guts" and try to come up with reasons for why that feeling is correct.

In truth, if there is no inbreeding between immediate family, nor continued inbreeding (over several generations) between extended family, then there is no concern over healthy offspring. If the relationship is consenting and all involved are satisfied, then it is immoral to prevent it, as it treads on the rights of the participants.

So: The "consenting adults'' rule applies. The "gross" feeling you get is not a substitute for proper moral reasoning.