Shana Decree, 45, and her 19-year-old daughter, Dominique Decree, allegedly carried out a killing spree in their two-bedroom apartment in Morrisville, Pennsylvania, about 30 miles northeast of Philadelphia.
Police discovered the apartment in disarray with the gruesome scene during a wellness check Monday afternoon around 4 p.m. local time.
The victims were identified as Shana Decree's children -- Naa'Irah Smith, 25, and Damon Decree Jr., 13 -- as well as her sister -- Jamilla Campbell, 42 -- and Campbell's 9-year-old twin daughters, Imani and Erika Allen, according to the Bucks County District Attorney's Office.
Campbell's 17-year-old son, Joshua Campbell, whose whereabouts were unknown Monday evening, has been located and is safe, according to the Bucks County District Attorney's Office. He was not at the apartment at the time of the murders, Bucks County District Attorney Matthew Weintraub told reporters Tuesday
There are no other suspects, Weintraub said.
"I just feel very confident that we have the two killers locked up that committed these atrocious, atrocious murders," he added.
Eight people, including both of the suspects, were residing in the small apartment, according to Weintraub.
Authorities have not disclosed a motive nor the causes of death. The case is still under investigation.
"We're working on some theories, but it wouldn't be fair for me to speculate," Weintraub said, "so I'm not going to."
During questioning, Shana Decree told police that everyone at the apartment "wanted to die," according to the affidavit of probable cause.
Dominique Decree, who had visible injuries to her neck, asked police if her family was alive and "repeatedly advised her wish to die," according to the affidavit.
"This is a terrible tragedy. I just spoke with the family of the five deceased, and we're all heartbroken," Weintraub told reporters at a press conference late Monday night. "I wish I had better news to report."
Shana Decree was arraigned early Tuesday morning and is being held without bail. Her daughter was expected to be arraigned later in the day.
They each face five counts of criminal homicide and one count of conspiracy to commit criminal homicide, according to the Bucks County District Attorney's Office.
"I wanted to come out here," Weintraub said, "and make sure that everybody knew that the people that committed these atrocious acts are now in custody and will be made to pay for their crimes."
It's unclear if either suspect has retained an attorney.
ABC News' Darren Reynolds contributed to this report.