Wendy Blackstone was deaf and legally blind, the newspaper reported.
"Homicide detectives believe the motive was Jill's frustration of being forced to provide Wendy long-term care, as well as the associated financial hardship," a news release from the LAPD said.
Wendy Blackstone died of the combined effects of Xanax and inhaling the gas that built up in the garage, the Los Angeles County Coroner's website states. The manner of death was still listed online as undetermined Monday morning.
The Los Angeles Times reported that Wendy Blackstone's autopsy report indicated that her death was "very suspicious" and "possibly staged." When she was found, an alleged suicide note was in her hand.
Investigators believed that Jill Blackstone wrote the note.
Near Wendy Blackstone's feet was a Weber grill and an ash-filled trash can, both of which were considered to be sources of the carbon monoxide, the Times reported. A nearly empty vodka bottle was found nearby, though the autopsy showed no alcohol in Wendy's system.
Detectives found several other notes near her body with end-of-life instructions, including a “do not resuscitate” order, but none of the notes had been signed by Wendy Blackstone.
Jill Blackstone later told detectives that a friend called her home and she "just 'came to' and did not remember what happened," the Times reported. She also told investigators that the grill was used in the garage to keep it warm.
She also said the charcoal was used to roast marshmallows, the newspaper reported.
ABC7 reported that Jill Blackstone was believed to have set the scene up to look like a suicide or accident. The notes and Blackstone's inconsistent statements led detectives to suspect her of killing her sister.
She was arrested shortly after the fire on suspicion of murder, but the Los Angeles County district attorney ultimately decided against filing charges, the news station reported.
The district attorney ordered further investigation into the case just days later, the Times reported.
Homicide detectives and arson investigators from the Los Angeles Fire Department conducted separate probes in the case, the LAPD news release said.
“The investigative follow-up required for a successful prosecution included significant travel, research, numerous interviews and additional forensic evidence processing, which added time to this extremely complex and sensitive investigation,” the news release said.
The district attorney filed murder and animal cruelty charges against Blackstone last month. Homicide detectives tracked her last week to a relative’s home in Hoboken, New Jersey.
When they spoke to her lawyer, the attorney told authorities that Blackstone was hospitalized in Baltimore for treatment of a medical condition, LAPD officials said.
She was taken into custody Wednesday at Johns Hopkins Hospital. As of last week, she remained in Baltimore Police Department custody, awaiting extradition to California.