If the wellbeing of offspring really is the issue, I don't see any way we can give any more scrutiny to incest than to other unions that are more likely to produce children with health issues. Either the possibility of producing or passing on genetic defects and health problems is a factor, or it isn't. It doesn't suddenly become a factor just because they're siblings, and I don't think it'd be reasonable to proscribe that other non-sibling couples be celibate/not procreate if their union carries similar risks. (Though perhaps if studies showed that incest was the greatest risk factor, that would be a pretty good justification for prohibiting incest.)
Parent-child incest is another issue entirely, as it's necessary for parents to have certain legal obligations if enforcing parental responsibility is going to be a thing. Same reason we place restrictions on teacher-student relationships, except in this case your role as parent never expires. The balance of power and responsibility means that these relationships are inherently abusive (and the child reaching the age of consent can only do so much to counteract that dynamic).
Siblings obviously don't have the same obligations toward one another. It really does boil down to society considering sibling incest "icky," and of course the offspring health issues problem (though, as I said before, that's not exclusive to incest).