What’s wrong with incest?
This a thorny question. Perhaps first of all because the taboo against it has roots in organized religion, and lies not primarily on a scientific determination, though--yes--mating with a relative may well have deleterious consequences in an offspring. so there is a strictly scientific aspect. "For my money" religious traditions often have a large component of shame and blame, and particularly related to sex, along with plenty of problematic dogma, which probably explains why I value the embrace of a spiritual path, but--for the most part--spurn organized religion.
As I see it, the main issue is the psychological problem of how an adult cohabitating and mating with a young person wields an unequal (even enormous) power and control over that individual....and the relationship may easily have the flavor of abuse. Even with the best of intentions, family relationships can be very psychologically "gamey" or manipulative (as in Eric Berne's psychological games in "Transactional Analysis") and do damage to one (or even both) of the participants. This general problem is also dealt with in "Object Relations" theory. In dysfunctional families, one often observes "co-dependency" (or, in OR terms, "unhealthy merging"), something which can be very pathological (and have a "yucky" feel, even apart from any sexual component.
This relates to the fact that part of becoming a mature, well-balanced person, lies in gradually moving away from the primary merged relationship with the mother, into independence and flowering as a complete person and then, generally, coming to comprehend boundaries and roles between oneself and others, and being able to effectively and fully function. With incestual relationships (and perhaps even with those between two parties with a large age difference), these categories (and "social rules") can become very confusing (and even "crazy-making"), even if not overtly abusive.
Some of the same issues are dealt with in a powerful mystical tradition called the "Diamond Approach" developed by H.L. Almaas (with principles that are similar to those of Sufism and Buddhism).
Now, is incest a sin? For me, no, not exactly, and I would urge you to dispense with any RELIGIOUS slant on the question. Moreover, I don’t consider it to be INHERENTLY a problem. Yet, it generally would be a mistake (more a matter of inappropriateness) because of the psychological problems outlined above, which frequently present, and I would strongly recommend against it. Put another way (and in remembrance of Robert Heinlein's novels), some very mature individuals might be capable of surmounting these problems, but the people in this category would probably be a small group. Finally, to engage in this kind of relationship might lead to complicated and very problematic legal consequences. Better to steer clear of it. Of course, the genetic proximity could be a consideration. I'm not sure I would be bothered by a healthy relationship between, say, second cousins (and even less should the couple practice birth control).