The same pesticide was also believed to have been responsible for the recent deaths of two children in northern Alberta.
Coroner Renee Roussel said Monday the fact Audrey, 20, and Noemi Belanger, 26, were both very sick and appeared to have the same symptoms led her to conclude they suffered the same health problem — intoxication.
Roussel's report describes phosphine as a pesticide that "kills everything that lives, everything that breathes."
She said it is officially forbidden to use phosphine to fumigate hotel rooms in Thailand, but that it might in fact still be used.
No trace of phosphine was found in the women's blood or their vital organs.
The parents of the two women attended a news conference in Riviere-du-Loup, Que., where Roussel released her report.
The coroner said they wanted to be present "to turn the page and to continue their grieving in private."
The sisters were from Pohenegamook, in Quebec's Lower St. Lawrence region.
Roussel said autopsies conducted in Bangkok four days after the deaths indicated there were no traces of violence or trauma on the bodies, but it was evident that the two had been vomiting.
Pathologists in Thailand concluded that, based on samples taken from the sisters' hotel room, the insect repellent DEET was responsible for their deaths.
But Roussel disagreed with that conclusion. She said the concentration of DEET noted in a Thai report was not toxic or deadly.
A Canadian government website says registered insect repellents containing DEET can be used safely when applied as directed.
In Alberta, a toxicologist has suggested phosphine gas was responsible for the deaths of an eight-month-old girl and a two-year-old boy in Fort McMurray. RCMP and fire officials said a woman took her five children to the hospital after they started vomiting.
One official said the family had recently brought a type of aluminum phosphide back from Pakistan and the tablets were placed around their apartment, particularly in one bedroom, to try to kill bed bugs.
But at one point, the mother vacuumed the home and disturbed the pellets, causing them to break down and get into the air.