Based on the erotic game by Dwarfsoft.

Tsukikawa Keisuke had a wet dream about his little sister. Saki was still very naive, so much that she thought babies were made by kissing.

In the dream, Saki was very unlike herself. She was experienced, seductive, visiting his bedside and intently removing his pants to expose his raging erection. Her fingers went up and down his shaft gently at first, the she tightened her grip, pumping her fist on his hardness. When he at last clenched and released, spewing an arc of jism, her mouth was expertly positioned to catch it all. She swallowed. Keisuke lay paralyzed on the bed. He felt awake, and ashamed. It was that sort of dream, so vividly graphic that he thought it could be real.

The next morning Saki greeted Keisuke in her usual way. Yes, it was just a dream. What a relief.

Keisuke learned that his father was home for the first time in a while. His father was a folklorist, like a cultural researcher. The timing was not a coincidence. His father explained that inside Saki lurked a demon -- a succubus. No... ... in fact it was more like Saki's true nature was a succubus, but she didn't know. Keisuke's father imparted this secret knowledge only to Keisuke. Beware the succubus, for once she tastes the seed of her prey, she desires it uncontrollably. When she transforms in the moonlight, she makes her seduction!

Keisuke loves his tender, simple little sister. But now, he's also tempted by the vixen.

Keisuke faces a dilemma: which version of Saki will he be with?

(Source: DLSite, translated)