Probably bc people mostly correlate incest with children molestation.

If we are talking about a mutually consensual relationship, most probably neither of them would go around talking about it since it odd, so society hasn't had enough cases to change the social status quo.

The other point you bring about health isn't relevant now, since I really believe it's just about social perception.

Pic rel. Me and sis, when we were little

My father abandoned us when I was 1yo.
At 8, I got a step father and later a half sister. But I just played vidya, I went to uni and when I got back I realized how much I miss my family while being sad of how much I missed out on life with them.

I want at least 2, a girl and a boy, girl first, I wanna make up for when I just ignored my sister. And then a boy to go play in a park every weekend, like I wish I could have