Caleb Harrison, 41, was found dead last August in a home in Mississauga, Ont. — the same home where his mother Bridget Harrison, 63, died in 2010.
At the time her death was classified as suspicious, but when officers started investigating Caleb Harrison's death, they determined both were homicides and both mother and son were asphyxiated.
Police have now also reopened the investigation into the 2009 death of Caleb Harrison's father, William Harrison, who also died in the same home.
Investigators originally determined he died of natural causes, but they are taking another look and are treating his cause of death as undetermined.
"When Caleb Harrison was found we were going to do a complete 360 on his life and on the life of the family and basically that investigation led us to where we are today," said Peel Regional Police Insp. George Koekkoek.
Melissa Merritt, 33, and her common-law husband, Christopher Fattore, 36, were arrested near Bridgewater, N.S., where they moved shortly after Caleb Harrison's death.
The couple will appear in court Wednesday morning in Halifax and is expected to be sent back to Ontario to face two counts each of first-degree murder.
Merritt and Harrison had two children and split up in 2005, which is also the year she and Fattore met, Koekkoek said at a news conference.
Merritt was previously convicted of the parental abduction of the children she had with Harrison — an abduction Koekkoek said occurred around the same time as Harrison's father's death.
The two children were living with Harrison at the time of his death, Koekkoek said. Fattore and Merritt also had children of their own and all kids are currently being cared for, Koekkoek said.
Merritt and Fattore were living in a farmhouse near St. Marys, Ont., when it burned down in March 2012, according to local news reports from the time. Fattore appears to have set up a fundraising website to help his family — which he said included five children between the ages of 10 and one with another on the way — rebuild their lives.
Fattore told the Stratford Gazette the fire started in the living room and foul play was ruled out. Koekkoek said Tuesday the fire has no relationship that he is aware of to the Harrisons' deaths.
A blog post under the name Chris Fattore from Nov. 8 titled "starting over" says he and his family, which includes six children, moved from Ontario to Nova Scotia.