Anta wo Nurasu no wa Ore dake Dakara

Yuka and Itsuki's parents have gotten married, and now they're step-siblings. Yuka's been trying to act as a loving older sister should, but she can't figure out why her stepbrother is so cold to her. Then, one day, Itsuki runs into her new boyfriend when she brings him over, and that's the last straw. Suddenly, he's all-male.