Lisa Dudley, 37, and Guthrie Jolan MacKay, 33, were both shot and their bodies left inside their home in September 2008.
Three people are already facing charges in connection with the deaths, and the province's Integrated Homicide Investigation Team is scheduled to hold a news conference Monday morning to announce more charges.
Last week, Bruce Main, 66, pleaded guilty to manslaughter and was sentenced to seven years in prison.
Jack Woodruff, 53, pleaded guilty to two counts of first-degree murder last year and was sentenced to life in prison without parole for 25 years.
Justin MacKinnon has also been charged with two counts of first-degree murder.
The case has generated controversy because Dudley and MacKay were found inside their home four days after neighbours called 911 to report hearing shots fired.
The RCMP officer who was first called to the scene never got out of his car, reporting he didn't see anything unusual. In a police review, he was later found guilty of disgraceful conduct and docked one day's pay.
Dudley was found tied to a chair and bleeding from bullet wounds. She died on the way to hospital. MacKay was found dead in the home.
Investigators have not released a motive or other details about their deaths, except that they were killed in targeted hits.