Two men on a motorcycle threw acid on the women and sped off in Tuesday's attack in Shamli town in Uttar Pradesh state, said Arun Kumar, the state's director general of police.
The sisters were hospitalized with serious burns. They're all in their 20s and teach at the same college.
No arrests have been made so far, Kumar said.
It was not immediately clear what provoked the attack, he said. Police were waiting for the women's conditions to improve before speaking to them.
Shamli is about 135 kilometers (85 miles) north of New Delhi.
In recent months, there has been growing outrage across India over violent assaults on women.
The fatal gang rape of a young woman on a New Delhi bus in December spurred the government to pass tough new laws for attacks on women, including a 10-year maximum sentence for acid attacks.