If two fully consenting adults engage in incest, and one or both of them are most definitely infertile, and neither of them are in any other romantic or sexual relationship requiring loyalty, is it then morally wrong?
As long as it's not doing any harm, it's likely more about societal norms than about an intrinsic problem. Though one thing worth mentioning, at least some forms of incest pose some doubt in terms of the consent part. Like a parent is obviously much older, a legal guardian for the child, their prior relation could be considered grooming and in general they could exploit parental knowledge and feelings. Depending on the indoctrination of the importance of "the family" they might be discouraged to talk to outsiders about that and so on. Though the stigma and the secrecy following from that might make that even worse.
The answer depends on how much you take responsibility for others mimicking you, and general wider context. If you just incest, but no children come out of it, you may argue that nothing bad happened. But if you think others will follow your lead, some of them may have "accidents" leading to unnecessary suffering. And that may be considered s moral wrong, because it mislead people into doing harm. Would it make doing incest privately a bad thing? Again it depends on wider context. If you argue that it's your life and you want nothing more than maybe no harm is done. But if you think that there's a moral good in having a family, having a heritage, then incest is morally wrong, because it's an empty pleasure that wastes your time. Such as drugs, or video games, or movies... Any time you're not productive, you don't make yourself stronger to fulfill your goals or save others, you waste a day.