ERIE, Pa. (CBS Local) — A Pennsylvania woman who weighs about 300 pounds has pleaded guilty to killing her boyfriend, partially by lying on top of him.

Windi Thomas, 44, admitted Monday to killing Keeno Butler, 41, last March at the couple’s home in Erie by stabbing him, hitting him with a table leg and crushing him under the weight of her body. Butler weighed about 120 pounds, the Erie Times-News reports.

Thomas faces 18 to 36 years behind bars after pleading guilty to third-degree murder. She is slated to be sentenced Dec. 21.

Prosecutors dropped charges of aggravated assault, reckless endangerment and possession of the instrument of a crime in exchange for Thomas’ plea.

“It’s a good result based upon the facts of the case,” said Thomas’ court-appointed lawyer, Mark Del Duca of Pittsburgh. “I think it’s fair for both sides.”

But Butler’s sister, Sandra Butler, says her family was hoping for a longer prison sentence for Thomas.

“I feel like she should have 40 or more,” Butler said. “She deserved life.”