Sanjana Devi, 22, and Gungun Kumari, 3, were killed Saturday at Gulab Bagh, about 50 km from Patna.
Their bodies were cut into pieces and thrown into the Ganges near Barh, Patna Senior Superintendent of Police Amrit Raj said.
Her family was not in touch with Sanjana after she married Dilip Kumar, who belonged to a different caste, four years ago.
According to police, Sanjana's brother telephoned Munna Kumar her two days ago and told her to forget the past and come to their parents' house.
Police suspect that Munna strangled her there and killed her daughter too.
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