My problem with incest (and I do believe this is a rational objection) is that it's often not between two consenting adults. Generally, you will find one party is younger and more impressionable and sometimes misunderstand their feelings for a sibling/family member which are manifested in an incestuous way because of pressure from the older party.
Apologies, I appreciate this is fairly anecdotal but I'm a little too busy to find some concrete stats and figures, but I feel it is a logically likely situation. Certainly in cases where the people involved have known each other for years, it's extremely likely there will be an age gap, and entirely possible that the younger person has been exploited.
As a concept, I don't have a problem with heterosexual relationship provided no one is being taken advantage of, if someone is then I morally have a problem with it. The same goes with incestuous relationships and I think, almost by definition, it's a lot more likely that someone is going to have their emotions manipulated.
If that isn't the case and its an instance between 2 adults (and has always been between two adults) and both adults are clearly consensual, then I don't see why it's anyone's business.