Maybe it's even a clash of social values.
If you think about it, why is love/sex between siblings shunned in the first place? It was a biological taboo, because of inbreeding. Thus it became a social taboo, too.
If we purely watch at the sexual standpoint the core reason to not have sex with your siblings or near family is somewhat gone. We have birth control now, if done properly the
risk of impregnation is low. So the social stigma still stands, it is gross to have sex with siblings, because, in most cases, you have known your sibling from when it was small.
I think in that regard it is the same "gross" like when a 18yo knows a person since it was 10yo and then wants to marry when they are 26 and 18 respectively.
In "normal" circumstances it would be considered a little far away but normal, while the constallation "I knew him/her since he/she was 10" holds a social stigma.
Now what is clashing here?
It's just an assumption, but maybe for some people it is not just a taboo-fetish to imagine sex with a family member, maybe in the light of birth control it's more
along the lines of an easy way to get sex and sexual experience. Your sibling is right there, has a pussy/penis, is maybe just as horny as you if you are roughly the same age
and except from the risk of "true love feelings above sibling love" and "being discovered" there are no real downsides to it anymore.