I'm in a charitable mood tonight, because I'm going to see Cowboy X and Metric tomorrow and I plan to be drunk with my friends for a good 36 hours or so afterwards, so I'm going to ever so daintily let the general tone of your post pass, and assume you're trying to start a real discussion of the difference between a homosexual and an incestuous relationship. You're welcome.
First of all, an incestuous relationship can never have a healthy dynamic because it cannot progress beyond the established roles of related family members. The relationship established with your family is a relatively static one. Romantic relationships must offer both partners an equal, consensual role. This won't happen with say, a big sibling and a younger sibling, not really. Second of all, it may sound stupid, but the issue of inbreeding is a big one. I know this, because I once observed an incredibly disturbing, and totally in earnest, discussion between members of a certain forum and one member who was engaged in such a relationship. By God, was it creepy.
Incidentally, if we're going to have a discussion about homosexuality as a natural or unnatural impulse as opposed to incest, which feels like where this is heading, then I may have to disillusion you about the animal kingdom. And indeed, queendom.