No, not the character from 'Sanford & Son'! Honestly.....
In the main Toobworld, Miss Jane Marple was portrayed by the late Dame Joan Hickson. She was a very staid, quiet woman who had a keen eye when it came to human behavior.
But that doesn't mean she was influenced by anyone in her family to become that way while growing up.
Joan Hickson played another Agatha Christie character in Toobworld who was not as... reserved as Miss Marple. She was more likely to be drinking a martini rather than tea.....
In the adaptation of "Why Didn't They Ask Evans?", Mrs. Ethel Rivington was a friend of the late John Savage. And we can see from her behavior that she may have been a bit of a lush. She more than likely kicked up her heels quite a few times as a younger woman.
The story takes place in the mid to late 1930's, so Mrs. Rivington wouldn't so much be an identical cousin as she may have been Miss Marple's lookalike auntie. Why not?