The two convicts, serving life term for raping a 15-year-old girl in 2008, were out on bail when they launched the brutal acid attack on the family members of the rape victim at Dharpa village in Khurja area.
Police said that the convicts, Ballo and Kaloo, along with two others forced entry into the victim's house and poured bucket full of acid on sleeping members, including her parents and two brothers.
While the girl, her father, and one of her brothers sustained minor burns, her mother and the other brother suffered serious acid burns and have been sent to Delhi.
The two convicts, who were released on bail on an Allahabad High Court order, have been arrested along with one of their accomplices.
The police is on the look out for the other person involved in the acid attack.