NORRISTOWN, Pa. (AP) _ Two sisters wanted to end quietly the life of their 90-year-old mother, whose brain was so ravaged by Alzheimer's disease that she spent much of the time screaming.

The sisters also wanted to die. An overdose of the mother's heart medicine, mixed with baby food, worked on the mother and one of the sisters.

The other sister, Bernice Jamison Cotlar, survived to plead guilty Monday to third-degree murder. She faces up to 20 years in prison.

Neighbors and friends testified that the 64-year-old Cotlar, a widow who fought depression, was the lone relative watching over her mother, Ethel Jamison, and 69-year-old sister, Leslie Jamison, who had a bent spine, deformed mouth and failing sight.

''The pressure was tremendous,'' said Cotlar's cousin, Arlene Silverstein. ''She had absolutely no help.''

Cotlar lost her composure in court when she described her mother's condition. She said her mother rarely knew where she was, and nursing home attendants would strap her into a seat, where she would scream continuously.

''I rubbed her head. I sang her lullabies. I rocked her seat,'' Cotlar said. ''I leaned close to her. I said, 'Mom, take my strength.' It was so hard to watch.''

Cotlar testified that she called an out-of-state doctor with a question about suicide and was told an overdose of prescription drugs would suffice.

Then, she said, she went out and bought three times the normal prescription amount of her mother's heart medicine. The doctor was not identified in court.

On July 30, she testified, she checked her mother and sister out of the nursing home where they both lived. The sisters tucked their mother into her old bed at home, emptied the medicine capsules into baby food and spoon-fed it to her.

''We were crying, telling her we loved her and how much we cared for her,'' Cotlar said. ''Leslie was on one side. I was on the other. It wasn't long. Then it was over.''

Then Leslie ate some.

''I saw my sister die. A little bit of spitting up, and then she was peaceful,'' Cotlar testified.

Cotlar also took some, but vomited. Then she took sleeping pills, but awoke hours later. She called police.

''I want to die, too,'' she told arriving officers. ''I had to do what's done, now let me die.''

Several charges were filed against her, including helping her sister commit suicide. But prosecutors dropped those charges in exchange for her guilty plea to the murder charge and an admission that what she did was wrong.

A sentencing date has not yet been scheduled.