@3ds1234
Lmao, I blame both all the manga reinforcing that idea the last 10 years and the weirdly huge amount of step-sibling porn on the internet, as well as how much of a meme it is.
But just for this series, I mean, when they introduce a character by doing the cringey 'naked apron' and being both super housewifey and doing the clingy "onii-chan" thing to end the chapter, it seems like they're being somewhat serious about it. Thankfully it was played off this chapter as another weird joke in an otherwise wholesome relationship