Keita has two older step-sisters, Ako and Riko, but since they aren't related by blood, they love him in a lustful way. After a mishap at school, Ako and Riko finally confess their love to him. Keita dislikes the thought of seeing them other then brother and sister, but as he tries to enter the same school as his sisters, he slowly becomes attracted to them.
This anime is very good, it's the first anime series in a decade that i watched from beginning to end without or very few lag time. Lag time meaning, those those times where you want to skip a whole selection because it got horribly predictable or boring. Not since samurai x ova had i had such a feeling.
While it's true that it's a adolescents boy greatest fantasy, and i don't have a sister so i would think that people that have sisters would feel maybe a bit differently about this anime. Nonetheless the plot wasn't that great, but it's better than 99% of the trash animes out there and it was EXECUTED very well, almost perfectly directed.
I have seen very hardcore anime hentai that bores the fuck out of me, this is the only anime that i have seen besides a true hentai, that the main characters has the balls to touch girls and not shy away like a common Asian kid in a horrible anime.
In summary it's a old concept but executed in a way where i would think it's fresh and very fun to watch. I think people might hate this anime because the main character is too realistic and actually wants to touch girls, unlike a common teen trash anime where the main character goes all red and runs away.