The two-year-old boy died Thursday while being treated at Stollery Children's Hospital in Edmonton.
Imam Sherif El Sayid of the Al Rashid mosque told mourners about the boy's death during the funeral for his eight-month-old sister, who died Sunday.
"He said today we are burying her and tomorrow is the funeral of her brother," an official from the mosque said.
Allan Vinni, who is the lawyer for the Fort McMurray family, identified the boy as Zia-Ul Hasan Syed. His father is Syed Habib and the mother is Nida Habib.
RCMP have said the mother took her five children to the hospital in Fort McMurray on Sunday after they started vomiting. The baby girl died and two of the children were transferred to Edmonton for treatment.
Taj Mohammed, principal of the Fort McMurray Islamic School, said one child, believed to be about five years old, remained in hospital.
"All we know is that the second child has passed away," he said. "The other one is still critical. The two who are with the parents are doing well."
Mohammed said the parents are struggling to come to terms with what has happened to their children.
He said people are doing what they can to help the family, which moved to Alberta from Ontario a few years ago.
"This family needs support. They need all the prayers that all the people can give."
The family had recently brought a type of aluminum phosphide back from a trip to Pakistan. The green tablets were placed around the apartment, particularly in one bedroom, to try to kill bedbugs.
Aluminum phosphide can cause long-term damage to a body's liver, heart and kidneys.
The Canada Border Services Agency and RCMP have said they are investigating the case.
According to Health Canada's website, imported pesticides must be regulated under the Pest Control Products Act and bear a Canadian label.
The poison is listed in Alberta as a Schedule 1 substance, meaning its availability and use is restricted to commercial applicators and trained farmers with licences. Each province has its own classification system.