Bro is gonna think his sis is another girl... Isn't he?

to be honest. that trope is so funny to me, there's even one where someone thought their s/o was cheatin on them and turns out their mom is just really young looking lol

i don’t know if i’m thinking of the exact same manga as you but rojiura pussycat has that exact same trope but the way the same treats the uke after they sort the misunderstanding out has me acting like a crack addict, it’s so cuteeee