Lucy if you'd read of Sheila's very unhappy beginnings in her short life, I'm sure you'd understand why her ending had ended the way that it did. She was a totally abandoned child,more so than Jeremy had been,then to have her life dictated to and controlled had been more than she could bear,even as an adult with a family of her own.
There's an old saying about " the worm turning ",and it did for sure.
A lot of people in this predicament can turn their lives around with love,help and support,but Sheila was getting none of these vital links,hence her deep depression had deteriorated until it became too late for any intervention so a medical " cosh " had been given to which she'd fought against.