The four sisters suffered burn injuries after two men on a motorcycle threw acid on them as they were returning from school. One of the girls was brought to Delhi for treatment; she is now back at her place.
The brother-in-law accompanied one of the victims to the hospital in Delhi and was with the family throughout, the police said.
The incident took place in Shamli, around 100 km from the national capital, on April 2 when the sisters, three of whom are teachers, were returning home from a government school.
The youngest sister, the police said, suffered maximum burn injuries.
Attacks on women have topped the national agenda since December 2012 when a medical student was assaulted and raped by six men on a moving bus in Delhi. She died two weeks later of her injuries.
Public anger prompted parliament to toughen sex offence laws including doubling the minimum prison sentence for gang-rape, but lawmakers voted against making acid attacks punishable with life imprisonment.
Acid attackers can be jailed for eight-12 years depending on the injuries inflicted, but the offence is bailable.
Activists have asked that India should regulate the sale of an acid called "Tezaab" which is designed to clean rusted tools but is commonly used in attacks.