Luna and Celestia are sisters.

Naturally, they love each other. But as Luna overlooks her history, and overlooks thousands of years, a peculiar thing comes to light.

Luna was not always the regal Princess of the Night, Guardian of Dreams. She, and Celestia, too, had many different phases and identities. And every single time that Luna and Celestia had gotten... uncomfortably close, it had been while they were underneath some sort of influence.

Luna discovers that these substances were less responsible than she thought.

An entry into Incest is Wincest's Siblings contest. Link here if interested.