Bishoujo Ikenai Hatsutaiken

Main character stays at his aunt's house for the 3rd year. He calls a female cousin Saki to tutor him and soon falls under her influence and gets treated as a servant. Protagonist is going to lose virginity with his classmate Karin at birthday party. But Saki opposes the idea greatly and at creates obstructions at every step.

[From vndbreview]