What sister?? There hasn't been any mention of Keisuke's family in this entire manga, I don't think.
Oh, I must have mixed this up with another very similar manga....
I read so many manga every day that sometimes, the really similar ones with similar premises and characters kinda get a little mixed up.
There is another very similar manga except the difference is that the MC has an older sister and he's a bit of a d*** towards her as well..... but she's a minor character....