Suman (28), her daughter Laxmi (9) and son Parul (7), residents of village Bagla in Hisar, were given poison by Suman's in-laws, according to the complaint filed by her father Ram Kumar.
The three were admitted to a private hospital in a critical condition were they died, police said.
Suman used to have quarrels with her in-laws over a piece of land, Ram Kumar alleged.
She was married to Ramesh in 2002. Ramesh died of heart attack about two years ago, the police said.