POLICE have declared a crime scene in a quiet suburban street in Brisbane's south after two people were found dead.

A 54-year-old man and a 58-year-old woman, who are believed to be brother and sister, were found dead in separate rooms at a Sunnybank Hills home shortly after 2pm on Tuesday.

The pair lived with their frail 89-year-old mother, who found them and alerted neighbours, who called triple-0.

Their cause of death is unknown and post-mortem examinations are likely to be conducted on Wednesday.

It is understood there is no obvious cause of death.

Senior Sergeant Gary Aschenberger said the house was in a messy state when police arrived.

He said police were investigating all lines of inquiry.

"We're open to everything at the moment and nothing's been ruled out," he said.

"(The cause of death) is unknown until forensics have been through there and obviously autopsies will be conducted as well."

One Macbeth Place resident told The Courier-Mail they weren't concerned about their safety, but that the incident came as a surprise.

"This is a really quiet street and everyone keeps to themselves," one woman, who did not want to be named, said.

"We didn't know the people who live there, but it is a tragedy no matter the circumstances."