Veer Pal, a resident of Mayapuri colony mixed sulfas tablets in the food and served it to his son and three daughters yesterday and also consumed the poisoned food himself, police said.
The family members were rushed to a hospital by the neighbours when their condition started deteriorating.
Veer Pal died in the hospital last night, while his 14-year-old son Kuldeep died today, SSP Nitin Tewari said.
The three daughters Shital (12 years), Isika (10 years) and Shivani (5 years) were still fighting for their lives in the hospital.
A home-guard has also been posted in the hospital to look after the ailing girls, Tewari said.
Their mother, Ranjita (40 year) has committed suicide nearly a month ago.