The Plague of Doves / Maggie Peace
Maggie Peace is the older sister of Billy Peace and gets pregnant as a consequence of an affair with John Wildstrand who is married to Neve Harp. She is part of the plot where John kidnaps his wife to get the ransom money from the bank of which he himself is the president.
After Neve finds out, John moves in with Maggie, but she soon grows tired of him. She has her son, Corwin Peace, and takes claim of the house. John moves to the garage and is soon left with nothing, having lost his job at the bank, his wife Neve, his girlfriend Maggie, and his son Corwin.
Maggie was about nineteen when she had Corwin, who was born around 1952-53, which means that Maggie is from around 1933-34, being a year or two older than her brother Corwin.