Anthony Robert Harvey pleaded guilty to five counts of murder today at Stirling Gardens Magistrates Court in Perth.
The 25-year-old handed himself into police on the morning of September 9. The bodies of his wife Mara Harvey, daughter Charlotte, 3, two-year-old twins Beatrix and Alice, and mother-in-law Beverley Quinn were found in a Bedford house on the same day.
On Wednesday Mr Harvey admitted to killing his 41-year-old wife and children at their home on September 3. He told the court he killed Ms Quinn, 73 the next day, also at the house.
Mr Harvey has been remanded in custody ahead of sentencing later this year.