ITA. Since most of the responses seem to be along the lines of "Ewwww no" I'm guessing a lot of the people responding are just grossed out by it. Some people are just as grossed out at the thought of marrying someone of the same sex as someone they are biologically related to. It doesn't make sense to say that one couple can get married despite it grossing some people out and another couple cannot get married because it grosses some people out.
I hate to single anyone out but this is just one of many comments I noticed that would be getting flamed left and right if this were a discussion on homosexual marriage.
Some people are saying we are genetically programmed to not be sexually attracted to our blood relatives. This is true. Most of us are also genetically programmed to not be sexually attracted to members of the same sex, so I don't think that argument applies either.
Genetic disease is a legitimate concern but the risk isn't really high enough to warrant the banning of the marriage. Most of the concern comes about when a particular recessive allele is carried in the bloodline, raising the risk of giving birth to a child affected by the disease the allele carries. For this argument to be valid I think we would also have to say it should be illegal for people in certain populations that carry a high frequency of detrimental alleles to marry within that population (e.g. Tay-Sachs disease in Ashkenazi populations) because of the higher probability of passing on the disease. A recessive allele for a debilitating form of dwarfism runs in my family, so should I be banned from marrying anyone else with a family history of the condition, just in case I'm a carrier and my spouse is too? There are just too many double standards.
I've purposely left my personal opinion out of this and tried to look at the entire comparison objectively. I'm merely agreeing that some of those who oppose sibling marriage but support gay marriage seem to be encouraging a double standard.