The "social stigma" here is that it's potentially dangerous from a pregnancy and birth standpoint, the hate of that just covers all of incest itself.
Also just the general idea that being with a family member is a no-no. No idea when that came to be, but I'd guess it's been around a little while.
Also, I think you misinterpreted what >>154393260
said, or maybe I just misinterpreted it more, I think he was just saying that the idea of doing it with his sister is gross, because of something about her. I don't think if incest wasn't so taboo he'd be doing her right now.
Now, if he was like "Help, my sister is so hot but I can't say anything to her because incest is a no-no" I could understand what you said, but to me it just seems like he doesn't like her like that?