They may have had more children as Sheldon says, referring to his mother, "I was always her favorite."
According to Sheldon, she apparently taught her son how to be evil.
She was first mentioned in the game, Nicktoons Unite!, SpongeBob asks Sheldon what his mother would think of Plankton's actions and Sheldon says that he believes that his mother would be proud of him. He mentions that Sheldon was her favorite child. Later, he sarcastically says that his mother would be disappointed in him for being a villain.