Kiss x Sis
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 than brother and sister, but as he tries to enter the same school as his sisters, he slowly becomes attracted to them.
[Source: Anime News Network; emended]