Megachu!

Based on an adult game by Front Wing.

Kousuke is an everyday nice-guy college kid who suffers from a reoccurring nightmare where he's a rampaging sex demon. He just can't figure out why that is, as he sits down to play his favorite video games like Magical Girl Lape-chan. One day a beautiful Goddess named Fauna appears before him, not to grant wishes, but to destroy him! It seems Kousuke has the power of an evil demon sleeping inside of him. Luckily, Fauna decides on an alternate means of stopping the demon, which involves lots of sex with her and sexy older sister (and love potion maker) Jordh. Unluckily, there's Natsuki, a little devil with plans of her own for Kousuke.

(Source: AniDB)