The incident took place when a trader Pragyanshu and his family were returning to Lakhimpur late on Wednesday night after attending a wedding in Gola, police said.
While the driver was killed on the spot, the trader's mother-in-law, Ashoka (60) died on the way to hospital, police said, adding Pragyanshu, his wife, sister-in-law and father-in-law were rushed to Lucknow Trauma Centre in a critical condition.
His three children suffered minor injuries in the mishap, they added.