They said the incident occurred last night in Sriganganagar district when the family was returning in their car from Sadulshahar.
The attackers stopped the vehicle and brutally beat up the victims. Rajbala and her mother Parmeshwari Devi died on the spot while Jaswant Singh (33) was injured, police said, adding Rajbala's one-year-old son, Rudra, was found in the car.
Jaswant in his complaint claimed that his mother-in-law, Devi, knew the attackers and he had stopped the car on her request, they said.
The matter is being investigated and a manhunt has been launched to nab the perpetrators.