Constable Basant Kumar used his service rifle to kill Sandeep, 23, and his mother Roshini, 55, from point-blank range in Gaurad village of Sonipat district, 200 km from Chandigarh.
He also left Parkashi, Sandeep's aunt, critically injured in the firing in their house.
Basant had yesterday afternoon forcibly entered the house of Sandeep who had been recently married to Sarita. Basant was having an affair with Sarita and they had eloped once earlier.
Police officials said that after committing the crime, Basant fled from the spot in a car along with Sarita. Police said that another youth was accompanying him at the time of the incident.
Basant had been placed under suspension last month after he remained absent from duty.
Police officials said he had eloped with Sarita recently after the families of both the lovers refused to get them married. The two had come to Chandigarh but were soon traced by the girl's family who took her back and got her married to Sandeep April 15.