A three-year-old boy is in critical condition after being stabbed at a birthday party at his Oshawa home on the weekend -- in an attack that also killed the boy's mother and injured his father and brother.

Nathan Kelly is clinging to life at Toronto's Hospital for Sick Children, while his five-year-old brother Riley and his father are in serious condition after a family associate attacked them on Saturday afternoon.

Police raced to a townhouse in south Oshawa around 4:30 p.m. Saturday after receiving a frantic phone call from a woman about a stabbing at the home.

When investigators arrived, they found 29-year-old Rick Kelly, his 26-year-old wife Leslie and two of their children suffering from stab wounds.

Leslie died from her injuries. It is believed she tried to save her husband and children.

"Leslie stepped in front of Rick and she was stabbed multiple times in the chest," relative Dave McLean told reporters on Sunday. "She's a hero, I mean she saved him."

It is believed the suspect in the case, 47-year-old Gino Petralia, entered the home and threatened the family with a knife.

Petralia was later shot dead by police during a confrontation at an adjacent townhouse unit.

The province's Special Investigation's Unit is now investigating that incident.

Petralia's 13-year-old son Steven had been living with the Kelly family since the summer, when the boy's mother died of cancer.

The child is Rick's half-brother. He was unhurt in Saturday's attacks.

Neighbours of the Kellys said Petralia has a history of mental illness. CTV Toronto reported Petralia also has a lengthy criminal record that dates back to 1994.

It includes convictions for impaired driving, dangerous operation of a motor vehicle, causing a disturbance and repeated failures to comply with court orders.

Neighbours believe the Kellys were given custody of Steven because of Petralia's mental-health problems.

Autopsies on both bodies are scheduled for Tuesday.

The SIU expects to complete its interviews of six witness officers and the one officer who discharged his weapon during the confrontation with Petralia by the end of the week, SIU spokesperson John Yoannou told ctvtoronto.ca.

The SIU is a civilian agency that investigates circumstances involving police and civilians which have resulted in serious injury, sexual assault or death.

With files from CTV Toronto's Austin Delaney and Chris Eby