Dochinpira 15: Twisted Desires (1996)

The 15th in a live-action adaptation of the manga of the same name by Makio Hara and Tetsumi Doko. Jin has hooked up with a beautiful woman, Kaori, but is surprised to learn that the person he's making him orgasm with is his older sister, Tomoko. Tomoko suffers from multiple personality disorder because of the shock of seeing her former lover killed in front of her eyes.