UPDATE: A 19-year-old man has been charged over the murder of a Warwick woman and her 26-year-old daughter on Sunday.
The man was in police custody in relation to the deaths of Tamara Horstman and her 67-year-old mother, Maureen.
Major crime squad officers charged the man, who was arrested in Mirrabooka on Tuesday morning, with two counts of murder, and one count each of aggravated burglary and one count of stealing a motor vehicle.
He has been refused bail and will appear in the Perth Magistrates Court today.
Miss Horstman's 26-year-old twin brother, Nicholas, called emergency services to the Warwick home he co-owned with his mother and sister, but paramedics found the women dead.
The parents of one of Miss Horstman's friends arrived at the Felgate Place property about the same time to check on her after she had failed to show up at work.
Major crime squad detectives are appealing to anyone with information about the crime to contact police.
Nicholas was questioned by homicide detectives less than 24 hours after making the first desperate call to paramedics. Police confirmed a man was helping with their inquiries but this morning said he had been eliminated as a suspect.
Hours later, police confirmed they had arrested a 19-year-old man.
Forensic experts scoured the family's home for clues and examined Miss Horstman's dark blue Mazda 3, found parked on a verge in Mirrabooka more than 8km from the family's home.
Police revealed on Sunday night that it was missing but would not say yesterday whether they believed it had been taken by her killer.
They want to speak to anyone who saw the 2008 sedan, registration 1CZO135, or the person driving it, between 9am on Sunday and noon yesterday, when it was found in Pampus Court.
Miss Horstman's father, Gerrit - Mrs Horstman's estranged husband - seemed to be in the dark yesterday as he wrote on Facebook in Dutch about his daughter's missing car.
"That must be the same car that is in the newspaper. That is all that I know," he wrote.
Friends posted messages of support on social media for Miss Horstman's surviving twin, who was said to be close to his mother and sister.
"Hey man, just heard the news, all my thoughts and prayers from out east," one friend wrote on Nicholas' Facebook page.
Another said: "What shocking unbelievable news. May God give you and all those close all the strength in the world to get through this. My deepest condolences."
Mrs Horstman has lived in the Warwick home for about a year after buying it with the twins in October 2012 when she and her husband sold their Duncraig home.
Mr Horstman bought a property in Chittering this year but has not yet built a home on the semi-rural land.
A neighbour of Mrs Horstman, who asked not to be named, said her death was a tragedy.
"She was a very gentle, quiet and lovely lady," she said.
"It's terrible, they were a very quiet household and it is a tragedy."
Other neighbours recounted seeing Mrs Horstman walking her dog, one calling her a "devoted dog lover".
Mrs Horstman is understood to have recently adopted a rescued dog after her previous animal companion died.
Forensic officers yesterday carefully dusted railings leading to the home's back door and scoured a laneway next to their home.
It is understood forensic work at the property could take up to a week.
The second of the women's bodies was not taken from the house until yesterday afternoon.
It is believed Mrs Horstman had chest injuries while her daughter was found with head injuries.
Police have not revealed how they died or if they have identified any motive.
Anyone with information is asked to call Crime Stoppers on 1800 333 000.