The thing is, it has a basis in reality. There is A LOT of incest shipping in some shipping communities, especially if you have two brothers or two sisters, so it overlaps with yaoi and yuri shipping. This is part of the reason why shipping communities in general need to be burned with fire. See, for example, the RWBY fandom, which, among other things, likes to ship the main character Ruby with her sister, Yang. In fact, that is one of the examples given. And I quote:
"In a
poll
of 817
RWBY fans (held after Volume 1), 23 said their OTP was Ruby/Yang, who are half-sisters. The ship is known as "Enabler". It isn't uncommon to see Ruby/Yang in Pollination (Ruby/Weiss/Blake/Yang) fanworks, though many fans also just depict them as sharing girlfriends in that scenario."
I honestly think the shipping tropes in particular need to be burned with holy fire.