The youngest sister, 19, was admitted to hospital with severe injuries after two men on a motorbike splashed them with acid on Tuesday evening in the Shamli district of Uttar Pradesh, about 100 kilometres (60 miles) from the capital.
The sisters, aged between 19 and 24, were returning home from a government school where three of them work as teachers. The fourth is a student.
“The victims were walking together when two men on a motorbike made lewd remarks and the man who was riding pillion splashed acid on all of them,” Abdul Hammed, the senior police officer investigating the case, told AFP.
“The youngest sister suffered maximum burn injuries and she had to be rushed to a hospital in Delhi.”
Hameed said no arrests had been made and the motive behind the crime was unclear.
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 to 20 years, but lawmakers voted against increasing the punishment for acid attackers.
They can be jailed for eight to 12 years depending on the injuries inflicted, but the offence is bailable.
Campaign group Stop Acid Attacks accused the government of ignoring the growing trend of such assaults which are often perpetrated by jilted men or their relatives.
It has called for India to regulate the sale of an acid called “Tezaab” which is designed to clean rusted tools but is commonly used in attacks.
“Acid has become the cheapest and most effective tool for men to attack women in India,” said activist and victim Archana Kumari, who hails from northern Uttar Pradesh state where Tuesday’s attack took place.
“Why is the government not stopping the sale of acid? Why are they supporting a weapon that has the power to kill and ruin a woman’s life?” she said.
According to the London-based charity Acid Survivors Trust International, about 1,500 acid attacks are reported globally each year.
But many more victims do not report their injuries to the authorities and instead suffer in silence.
In 2011, neighbouring Pakistan adopted legislation increasing the punishment to between 14 years and life for acid attacks and a minimum fine of one million Pakistan rupees ($10,200).