"That would be like saying, well there's there are a lot of people who like to use drugs so let's go ahead and accommodate those who want to use drugs. There are some people who believe in incest, so we should accommodate them. There are people who believe in polygamy, should we accommodate them?"
How stupid do you have to be to jump to that conclusion?
The idea of a marriage is that two people are making the ultimate commitment of pledging their undying love for one another, forsaking all others, and taking care of one another for as long as they are allowed to live. In what way does this bonding have to do with a brother and sister having sex with one another?