Joan (Kate McNeil) was the secondary villainess from "Murder Murder," episode 3.11 of Diagnosis Murder (airdate February 23, 1996). She was the identical twin sister of Jennifer Stratton, who was married to wealthy Bill Stratton.
It was later revealed that Joan was a villainous conspirator in her sister's plan to kill Bill, which Jennifer does in the beginning of the episode. Following the murder, the evil Joan helped Jennifer cover up the crime and create alibis for both of them, with the original result seeing both sisters sharing Bill's inheritance. What Joan didn't count on was the sheer greediness of Jennifer, and she ended up strangled to death by her sister, who wanted the money all to herself.