I remember a VICE article from a decade ago picked up the trend and some people on neogaf attributed the sudden rise in popularity to the Lannisters in GoT. It made sense back then since the show and the spike in viewership for the genre both happened around the same time.
Maybe but they are full siblings in GOT, it’s straight incest. The step stuff seems more likely correlated with people’s real experiences growing up of mixed and split families. Idk for sure of course.