Sister Sheila was a nun in the same convent as Sister Ingalls and Sister Mary Eunice when they were both novices. She appears solely in Sister Ingalls' flashbacks in "Take a Break From Your Values". She is portrayed by Hayley Treider.
Sister Ingalls meets Sister Sheila and Sister Mary Eunice when they discuss stopping the war in Vietnam. Sister Ingalls seems interested in this, and runs over to ask what's happening. Sister Mary Eunice informs Sister Ingalls of the rally that is happening in Washington. Sister Mary Eunice offers a leaflet to Sister Ingalls as Sister Sheila stands by. Sister Sheila says that she didn't realise that nuns are allowed to read newspapers, Sister Ingalls agrees and tells Sister Mary Eunice to put the leaflets away as they do not have the veil. After Sister Mary Eunice says Sister Constance gave her permission to, Sister Ingalls and Sister Sheila agree to go along with Sister Mary Eunice.
After returning from the rally, Sister Sheila, Sister Ingalls and Sister Mary Eunice exit a Volkswagen van and thank the driver. They return, and Sister Mary Eunice reveals that they did not actually have permission from Sister Constance. Sister Sheila is visibly anxious about being in trouble, saying she has "fallen in with the bad nuns".