The deceased have been identified as Priti Devi and her son Balmua (3), Govindganj Police Station SHO Rajiv Kumar said.
The woman's brother Pradeep Kumar, a resident of Areraj, today lodged an FIR against five people, including her husband, father-in-law and sister-in-law, he said.
Sunita Devi, the sister-in-law, has been arrested but others are still absconding, Kumar said.
The bodies were sent to Motihari sadar hospital for post-mortem.