Though I would personally find it strange, I would have no problem at all with sisters marrying each other. A brother and sister is stickier because of the risk to offspring. If they were infertile or it was okay to abort then maybe. But then what if they, for some sick reason, actually wanted to bring disabled children into the world? Of course we already have this problem with non-related parents who refuse to abort fetuses with hydranencephaly and the like. So effectively it's an issue that is not specific to related persons and can be neutralized given additional conditions, such as the acceptability of abortion.Sisters marrying each other? Brothers marrying each other? Father and daughter? Father and son? Mother an daughter? Mother and son? Uncle and nephew? Aunt and niece? Aunt and nephew? ...
If they love each other, why not?
Where do you draw the line?
The additional problem with father and daughter is that there's a high chance there is an exploitative relationship at play. I would have to say that I would allow it if the daughter was an adult and was not dependent upon the father for material support, but there would be reasonable suspicion for abuse and they would have to be willing to speak to mental health professionals to ensure abuse is not taking place.
I think you need to brush up on your history and/or anthropology. The line has not been in the same place for thousands of years and it has not been the same between cultures. It used to be considered wrong for Blacks to marry Whites. It used to be unacceptable for peasants to marry nobles. And many cultures have sanctioned homosexuality in history.The line has been drawn for thousands of years, and it's obvious why.
So, where is the "New Line" and why?
What is a relatively new phenomenon in OUR culture is marriage for love. Marriage really did used to be about property (which often included the wife) and inheritance for the children that aren't bastards. Traditional marriage is hardly something to be proud of, but I can understand why it was that way. Women in particular did not have the luxury to choose based upon love as they were forced to be dependent. Anyway I'm drifting off-topic a bit:
While not unique to it, anti-homosexuality is largely a tradition of religions descended from Moses, which through genocide and dedicated missionaries came to be dominant religions. But it was just one of many rules, some arbitrary and some not, that Moses and his kind pulled out of their asses.