Because the law has stated that incest is illegal. Just as the law used to say gay sex is illegal. Just as the law currently says murder is illegal. INCEST IS ILLEGAL Get over it. If you want to make INCEST legal start your fight to make it legal, and get the **** off gays accusing them of promoting incest. It's a dumb ass argument.
You do realize that the law prohibiting incestuous marriage were the result of marriage being between members of opposing sexes only. You knew that right?
Now, after the federal laws are forced upon the population, the laws prohibiting same sex sibling marriage are no longer applicable. They become just another "tradition" your side threw away.
You realize that, Right.
What legal argument is there to stop teo members of the same sex, even closely related, from the rights and benefits of marriage?
Let's see if you have guts enough to answer, or will you join Seawytch and Skylar in there bigoted avoidance of the question.