The man, identified as Dhananjaya Mandal alias Naresh Mandal, poisoned his wife Sarita Devi and two children Kalpana (10) and Ramesh Kumar (7) to death last night after a quarrel with his wife whom he suspected to have an illicit relation with his cousin Niranjan Rai, Dhuraiya police station's Station House Officer (SHO) Dinesh Prasad said.
The cousin also consumed poison soon after hearing about the murder of his sister-in-law and her children, Prasad said.
He was admitted to a hospital in Bhagalpur.
Mandal has been arrested and charged with the murder of his wife and two children, the police officer said.