Good manga, but I didnt like, that it was just focused on the hate/ntr aspect, the guilt as a consequence and that you cant do anything about it, because you cant talk it out with a dead person. The author wanted this, but I would have appreciated it, if Haru would have realized, that her sister loved her romantically instead of Tougo, at least at this time. So Natsumi didnt really try to steal Haru's boyfriend, its just that her boyfriend didnt love her in the first place, but actually Natsumi. This part was left out in the end, because it wasnt revelaed in the blog. Natsumi actually didnt try to steal the boyfriend, her sister was stolen from him.
And yeah, Im sure the forbidden onesided love was real, because otherwise you would be happy for your sister, if she is in love with someone decent and would support her. But Natsumi felt uncomfortable everytime Tougo appeared, wanted leave the scene as fast as possible and zoned out everytime her sister doted on him.