The five died Sunday at multiple crime scenes in and around the town of Carman, 85 kilometers (53 miles) southwest of Winnipeg.
Police responded to a hit-and-run and found the woman lying dead in a ditch on Sunday morning.
More than two hours later and 70 kilometers (44 miles) to the north, officers were called to a report of a burning vehicle and found the children outside the car.
The children were pronounced dead at the scene and police took Manoakeesick of Carman into custody while the body of the teen girl was found later at a home in Carman.
Police said autopsies were being conducted and declined to provide further details as the investigation continues.
In Carman, police tape blocked off the front and back yards of the family’s small white bungalow, while forensic crews could be seen going in and out. Police vehicles were parked outside.
Children’s toys and a bike were strewn across the back lawn.
“This is a dark time in Manitoba,” Manitoba Premier Wab Kinew told the news conference.
Randy McFarlane, who lives a few houses down, said the home was a rental property. The family had lived in the house in the quiet neighborhood for more than year, he said.
“You never hear anything like that here and you don’t expect it to be next door,” McFarlane said.
The children would play in the yard, McFarlane said, and he never had any interactions with the mother. Occasionally he would hear her yelling into a cellphone and said something “seemed off.”
Carman Mayor Brent Owen said the whole community has been affected by the tragedy. He did not personally know those who died, despite being familiar with about 80% of the town’s residents.
“It’s just absolutely horrific,” he said.
The Prairie Rose School Division said crisis response teams were put in place for the Carman Collegiate and Ecole Carman elementary school.
Police continue to work to piece together how the deaths unfolded, Arseneault said, adding autopsies were scheduled.
“Young innocent lives were senselessly taken yesterday and we grieve with all Manitobans,” Arseneault said.