Gerald Imes said on "The Oprah Winfrey Show" that the molestation continued for a year or two, starting when he was 13 and Mo'Nique was 7 or 8.
"I abused and betrayed the trust of another sibling, my sister, my blood sister," Imes said. He apologized to the actress, saying "I'm sorry, Mo'Nique. I'm sorry."
Imes said he decided to appear on Winfrey's show to apologize to Mo'Nique and bring their family back together. Imes said he himself was molested and he was using drugs and alcohol at age 11.
"Hopefully somewhere, somehow as siblings we can come back together as brother and sister," he said.
Mo'Nique has discussed her brother's molestation in previous interviews. She hasn't responded to a request for comment sent to her publicist. Winfrey said Mo'Nique didn't want to be a part of the interview but gave Winfrey her blessing.
"She said if you're [Imes] expressing what you had done to her could save one family then it would be worth it," Winfrey told Imes.
Mo'Nique's parents also appeared on the episode.
"It was such a heartbreaking thing to accept," said her mother, Alice Imes.
Mo'Nique received the supporting actress Academy Award in March for her role in "Precious: Based on the Novel 'Push' by Sapphire."