So, as we all know, the U.S. Supreme Court recently legalized homosexual marriage nation wide. So a thought came to me, if two males or two females are allowed to get married on the basis of rights and equality, why can't say, a brother and a sister get married (hypothetical, I'm not into that stuff)? Aren't they equal too? Don't they have the right to get married because they love each other? 

I'm not here to criticize or draw lines in the sand or anything, sorry if I'm insulting you but it's not my intention, it's an honest question with no malice behind it.