In a couple of panels it's kind of looking like onee-san is starting to suspect things. Now it only remains to be seen what she's going to do about it. I wish this isn't the kind of realistic story where she's a responsible adult and will act accordingly (to cut their connection).
I kwon that its very posible that onee-san break his heart but I have faith :'v
(https://nhentai.net/artist/akagi-asahito/ THIS IS AN SPOILER I HAVE FAITH >:'V)