Everyone is talking about sister, but I really like the fact that Senpai was able to defeat his shyness and say that he is curious about Nagatoro. I think that this really is progress.
This is an interesting change-up because in the original picture gallery doujin, Nagatoro had an older brother- she uses him to fake NTR the protagonist. So now I wonder if he still exists or if he's been replaced with this elder sister, since I can't recall him being mentioned in passing in the serialized version.