I find it arbitrary
I've always wondered how the taboo around incest got started. I also think there's kind of a double standard about it: people are all about being good and righteous based on religion, but there's incest all over it, and all over history as well. They either ignore it or they don't even bat an eye. I understand the genetic implications of it, but from a moral standpoint, I think it's very arbitrary. In my family, our sex education was very open and revolved around three pillars: if it's consensual, responsible and pleasant, it's ok. So, as long as I take care of STDs and use contraception responsibly, I've never seen why not get sexually involved with anyone I fancy. And many of my family members would agree.
I, for one, lost my virginity to my older sister, and we're both very comfortable with that fact (even if we don't tell everyone we know about it). It happened after I discovered she and our oldest cousin were a couple (she was 23 and he was 31 by then, and I found that extremely hot somehow. I was 18). We had a very honest talk about it and concluded that same thing: they both wanted it, they took care of themselves, and they enjoyed it very much, so it wasn't wrong. After I told her I was attracted to her too, she offered to have sex with me. In doing that, I was welcomed into a free, accepting relationship not only with her, but also with other family members. turns out she often slept with other cousins as well, and so I did with our other female cousins. They're all older than me, so I've gotten to enjoy very pleasant experiences with them.
But no doubt, my sister is the sexiest woman I know, and I rarely enjoy sex with other women as much as I do with her.