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 bec