So, this is basically "Master Teaser Takagi" meets "Domestic Girlfriend". They are going to try to get the other one to slip up and act like a lover instead of a sibling.
The problem with that, who decides what constitutes a breech of sibling relationship? Her walking around in a bath towel should have been enough. Second, they are trying to keep this a secret from their parents, so why strive to have one show that they are more than just siblings?
From just the first two chapters, it seems to me that he doesn't really care about her at all, but she might still hold feelings for him.