The incident occurred when the victims' car rammed into a stationary truck, allegedly parked wrongly along the highway, 6 km from here, Sub-Inspector (SI) Jasbir Singh, In-charge of Urban Estate Police Post, said.
The deceased have been identified as Harpreet Singh Happy (25) and his sister Ashu (22).
While the brother-sister duo died on the spot, Ashu's daughter Pari (2), her husband Rimple, son Sidhu and sister Sahiba were rushed to a local civil hospital and from there they were referred to a hospital in Jalandhar in a critical state, the SI said.
All six were returning to Ludhiana after paying obeisance at Golden temple in Amritsar, Singh said.
Earlier in the day, the SI has said three persons, including Pari, were killed. However, later in the evening, he said the minor girl was among the four injured and admitted to a hospital.
A case has been registered against the truck driver, who is absconding, he said, adding the truck was impounded.