Manno Devi (14), Raj Kumar (17), Ravi (7) and their stepmother Soni Yadav (35) were injured when one Deepu threw acid on them in their Jamuniyapur colony residence, they said.
The injured were admitted to the district hospital, where the condition of two was stated to be critical.
Deepu has been arrested, police said adding, the matter was being further probed.