The story premise looks dark&grim
I don't know how this could be a comedy except if the main character breaks up early and starts dating her twin sister instead, and she keeps teasing both of them with it(and even that would be pretty nasty).
To be honest, it reminds me a lot of Nozoki Ana, but without an extreme peeping fetish and instead a teasing fetish, meaning the story can only be concluded if the main character ends up with Manatsu [these names, one's using summer and one winter]...
The main character is an idiot. He goes out with a girl but does nothing with her. That's why people usually start out as friends and only advance to a relationship if they have enough feelings for a physical relationship.
Since he was unaware of her sister, it is almost certain that he was never over at her house and never asked her any personal questions = no knowledge of the other person, which means that both of them are not even friends.
How they are supposed to have a lasting "relationship" if they are not friends is beyond me, and I guess he'll soon know her sister better than his "girlfriend" that is nothing more than a crush (i.e. physical attraction) in my eyes.
So, either the author, his editor and/or publisher are extremely lazy/dumb, or this is all foreshadowing how the story ends.
If this makes a 180 and the character actually ends up with Mafuyu, it only testifies as evidence of the incompetency of Japanese authors nowadays to write a good romantic comedy story <_<