A Philadelphia-area mother and daughter have pleaded guilty to killing five family members -- including three children -- over two days in their apartment last year, authorities said Monday.

Shana Selena Decree, 47, and her 21-year-old daughter Dominique Kiaran Decree will serve five consecutive life sentences after pleading guilty but mentally ill to five counts of first-degree murder and one count of criminal conspiracy, the Bucks County District Attorney's Office said.

Shana Decree, 47, left, and her daughter Dominique Decree, 21. The mother and her adult daughter were sentenced Monday to life in prison in the slayings of five close relatives, including three children, outside Philadelphia last year. (Bucks County District Attorney's Office via AP, File)

Shana Decree, 47, left, and her daughter Dominique Decree, 21. The mother and her adult daughter were sentenced Monday to life in prison in the slayings of five close relatives, including three children, outside Philadelphia last year. (Bucks County District Attorney's Office via AP, File)

Prosecutors said between Feb. 23 and Feb. 25, 2019, the pair killed Shana Decree’s children, Naa’Irah Smith, 25, and Damon Decree Jr., 13, both of Morrisville, her sister, Jamilla Campbell, 42, of Trenton, N.J., and Campbell’s 9-year-old twin daughters Imani and Erika Allen.

"Today, we mourn with the families of Erica and Imani Allen, Damon Decree, Naa'irah Smith, and Jamilla Campbell, who were all murdered by mother and daughter Shana and Dominique Decree," Bucks County District Attorney Matt Weintraub said. "Were it not for their severe mental illness, both would face the death penalty. As it stands, they both will spend the rest of their lives in prison cells separated from the rest of us, as punishment."

The deceased bodies were found Feb. 25 in an apartment bedroom in Morrisville. The twin girls, Decree and Smith were killed by homicidal asphyxia and Campbell died as a result of homicidal ligature strangulation, according to an autopsy.

A makeshift memorial is seen outside the Robert Morris Apartments in Morrisville, Pa. Shana Decree, 47, and Dominique Decree, 21, were sentenced in Bucks County Court after entering guilty but mentally ill pleas to five counts of first-degree murder. (AP Photo/Matt Slocum, File)

A makeshift memorial is seen outside the Robert Morris Apartments in Morrisville, Pa. Shana Decree, 47, and Dominique Decree, 21, were sentenced in Bucks County Court after entering guilty but mentally ill pleas to five counts of first-degree murder. (AP Photo/Matt Slocum, File)

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Shana Decree told investigators all five victims wanted to die, prosecutors said. Several health professionals said both suspects were guilty but experiencing mental illness at the time of the murders.

"By murdering, Erica, Imani, Damon, Naa'irah, and Jamilla, they’ve decimated entire generations of their own family," Weintraub said.