In yaoi, there's a lot of brothers with crushes on brothers or cousins or even uncles and fathers. But, yaoi itself is already a rather sexual genre, so it's not a stretch for someone to throw in their incest fetish.

Yosuga no Sora was so awful. I watched to the end of the first episode and if I remember correctly, it ended on a cliffhanger with the younger sister coming to the brother in her undies and going "Brother, make me come." I think from reading some reviews that it was something not as pervy if I'd continued watching onto the second episode, but I was so done right there.