The incident occurred around 5 PM at Sindhri when the family members were returning to Payala from Bilara in the car, they said.
The car collided with the trailer coming from opposite side of Gudhamalani at the particular site where the highway had no divider, they said.
The victims are yet to be identified, police said, adding no casualty was reported from the trailer's side.
While seven injured were rushed to nearby hospitals, four of them were referred to a hospital in Balotra, they said.