Many years ago I read a story (or a story and it's sequel) about a brother and sister who diving (scuba diving in the middle of the ocean) for treasure/archeology/something and find an old decorative comb, when the brother uses the comb to help his sister comb her hair he discovers it is a magic mind control comb that lets him plant suggestions in her mind, which he uses to make her have sex with him.
The second part the siblings are investigating (or possibly the brother just made his sister believe they are investigating) a possible organized crime link to a BDSM resort that requires them to go undercover in the resort and the brother uses the comb to convince his sister they need to pretend to be husband and wife and are constantly being recorded, even in their room so she needs to "pretend" to be his sub all the time.
