Superintendent of Police (SP) Chakresh Mishra said that Kamal Kant, a resident of Deoria village, attacked his daughter-in-law Shikha (27) and grandson Ayush (3) with a sharp-edged weapon.
He said that while Ayush died on the spot, Shikha sustained serious injuries and succumbed to injuries in the district hospital.
He said that Kamal Kant has been detained and quizzed.
"Prima facie it seems that Kamal Kant is mentally disturbed and committed the crime after he got disturbed with the child's cries," he said.
(With UNI inputs)