First of all, almost every case of incest is rape, not consensual, so to put forth some 1 in a million scenario is deviating from the debate, but the Right is very good at getting off issue in these debates. Would I find a brother and a sister getting married offensive, yes I would, as would most people you'd ask. But as for relevance of incest for the topic of gay marriage, it has absolutely no bearing other than to cloud the issue.