Chicago police said the two girls, both 13 years old, fell into the water at Leone Park Beach in the 1200-block of West Touhy Avenue at about 7:30 p.m.
Marisol Perez's brother had been with the girls when they entered the water.
"He just said that they were in the water playing and he didn't know what happened to the girls," she said.
A beach hazard warning indicating potentially life-threatening waves and rip currents had been issued for Cook County and Northwest Indiana, lasting through Friday evening.
Perez said her mother had warned her brother not to go swimming Friday afternoon, but he wanted to be with his friends.
Perez said the water quickly turned on the teens. Her brother and two friends were able to make it out.
"He's ok, he just scared," she said. "He doesn't know what's happening to his friends right now."
Rescuers said five police officers went in searching for the two teenage girls who had gone under. But the current was even too much for well-trained police officers.
Nelson Lovera swam to assist an officer he saw struggling.
"The water was really rough," he said.
The Chicago Fire Department said three boys had also been pulled from the water but did not require treatment.
Crews continued searching for the other teenage girl and found her about 20 minutes later. Officers ran the equivalent of three city blocks to where she was found. She had been underwater for at least 45 minutes. The Chicago Fire Department tweeted at about 8:50 p.m. that emergency responders were attempting CPR.
Cpr in progress on teen girl.— Chicago Fire Media (@CFDMedia) July 7, 2018
Both girls were taken to St. Francis Hospital for treatment. One was in critical condition. The other's condition had stabilized and she was expected to be OK.