The women were killed at Coquitlam, east of Vancouver, CKWX radio reported Monday.
One of the women and a male passenger had been in a sport utility vehicle crash Saturday and the other woman stopped with her husband to help. As they approached the wreck, a white Jeep Grand Cherokee sideswiped the SUV and killed its driver and the woman who was trying to help, police said.
The two women, 27 and 30, died at the scene. The man who was in the passenger seat of the SUV was taken to a hospital with serious head and leg injuries but was expected to survive, The Vancouver Sun reported.
Investigators found the Jeep abandoned about a mile away from the scene of the accident.
Police said a 37-year-old man was in custody. They would not release his identity until he is formally charged, they said.
The 30-year-old woman who had stopped to help was identified as Charlene Reaveley of Port Coquitlam.
Neighbors hearing of the incident described Reaveley, a mother of four, as a pillar of the community.
"She was a fantastic person, a fabulous mother, daughter and friend," next-door neighbor Joe Digiandomenico said.
Reaveley was like a second mother to all the children on the street, another nieghbor said.
"The kids in the neighbourhood, she'd take them over like they were her own." he said. "They'd play in the yard, she'd feed them. Even if she didn't know the kids, she treated them like they were her own."