They do it with Mirrors 2* - Julia McKenzie

Preview s4 ep3. Book aka Murder with Mirrors. Elderly Miss Jane Marple is asked by Ruth (Joan Collins), friend from dancing debutante years, to visit Stonygates and protect her fragile sister Caroline Louise. The confusion of three marriages and half-siblings is thankfully visually overpowered by posh peaceful estate and elegant dressing for dinner and all occasions. Hats, gloves, and perfect pearls.

Carrie is now married to Lewis Serrocold, one trustee for the Gulbrandesen Foundation to rehabilitate delinquent boys. Another trustee is (Carrie's stepson by second marriage) Christian Gulbrandsen. Edgar Lewis, ineffectual pompous, nominally Lewis' assistant, claims various famous fathers, then loudly accuses Lewis himself behind locked study door, shoots twice and misses. But at the same time in a guest room, Chris is shot dead.