The women -- all teachers -- were returning home after proctoring exams in the state of Uttar Pradesh Tuesday when the attack took place, The Times of India reported Thursday.
The two assailants allegedly used spray guns to hit the women -- ages 20 to 26 -- with the acid.
Three of the sisters and one passerby, who was also hit by the acid, were in stable condition, a family member told the Times.
The 23-year-old sister was taken to Sir Ganga Ram Hospital in New Delhi with severe burns on her chest, face and neck. Doctors said they were most concerned by her eye injury.
"The cornea has [a] severe acid burn injury. Blood supply to the area is restricted and even the surface area is affected," said Dr. A.K. Grover, chairman of the hospital's ophthalmology department.
Grover said doctors put a graft in the woman's eye to supplement nutrition and restore the surface covering.
The women's uncle told the Times the attack could be related to their prevention of students' cheating while proctoring exams.
"It seems unlikely that merely stopping examinees from cheating was the reason behind the attack though we are not ruling it out. We are probing [a] personal enmity angle," said an officer at the Kandhla police station, adding that no arrests have been made.