In the novel A Murder is Announced, Sonia Stamfordis (née Goedler) was the sister of Randall Goedler, and the mother of Pip and Emma. She quarrelled with her brother because he disapproved of her marriage to Dmitri Stamfordis, and the quarrel was never made up.
She and Dmitri split up about three years after Pip and Emma were born. Sonia took custody of Pip, while Dmitri took custody of Emma.
Sonia is not seen on screen in any of the TV or film adaptations of the novel. In Murder Party, the French Télévisions film adaptation of the novel, there is a parallel character also named Sonia with a slightly different backstory.