Police confirmed that Kevin, 55, Jennifer, 51, Cameron, 16, and Robert Dunsmuir, 19, died inside their home today.
They were found together in an upstairs bedroom, emergency officials say.
Another son, David, 24, is the only remaining member of the immediate family. The Brock University student was away at the time of the fire, and has been notified, police say.
Neighbours say the family had lived in that house for years, and Kevin Dunsmuir was a prominent lawyer in Newmarket.
Members of the community gathered at Sacred Heart Catholic High School in Newmarket, Ont., on Friday evening to mourn their loss.
Ashley Bailey, 16, told CBC News she knew the family well, and that neighbours were devastated to hear about the deaths. She described them as a well-known family in the small community.
"They were sweethearts. They were the nicest family ever," said Bailey. "I can't even begin to explain how many people are devastated over this."
She said she used to play hockey with the family's youngest son.
"I would be over at his house all the time. And I called Jennifer, 'Mama Dunsmuir'. I was like unbelievably close with her," she said.
Bailey added that Jennifer faced some health problems recently.
"She went through a really bad stroke, and it was a miracle for her that she was alive … So this happening is just terrible," she said.
York Regional Police said that at 5:30 a.m. ET they received a 911 call and were dispatched to the home on 72 Howard St.
Police were first to arrive on the scene, followed shortly by firefighters.
Const. Blair McQuillan says police and firefighters were unable to get three men and a woman out of a second-floor bedroom, where they died.
Fire officials said that most of the firefighters are part-time volunteer firefighters.
CBC News reporter Shannon Martin said neighbours were standing outside their homes on Friday in disbelief.
"People can’t believe this has happened in their community at the start of the holiday weekend," she reported.
"People are standing on their front lawns here, they’ve got their dogs, their kids and wagons and tricycles. People are milling around, they’ve lost a family on their street. This family has been here in this community for more than 20 years.
“We’re also hearing that the mother had some health issues recently and may have had some mobility problems."
Shawn Sekalala, a friend of Cameron's, first learned of the tragic news through friends and social media. He said he immediately drove to his friend's home.
"I was like, 'This can't be.' So I drive up here and started asking questions, and then I figure out that he actually died," he said. "He was like my brother, like my other half."
Ontario Fire Marshal spokesman Scott Evenden says the investigation into the deadly blaze has only just started and couldn't say where the fire broke out or if smoke alarms were working.
East Gwillimbury Mayor Victoria Hackson said she knew the people in the home personally.
"It's a very difficult day for us as we mourn the loss of four residents," Hackson said at the scene. "Our prayers and thoughts go to our community."
She said it was a "terrible loss of life," particularly at this time of year, in their tight-knit community.
"We are almost rural in many respects... we will gather all of those that are hurting and work to try to start the healing process," she told CBC News.