The alleged kidnapper's ex-brother-in-law, Jose Figueroa, tells RadarOnline.com that Castro physically abused his wife Grimilda for years, so much so, he claims, it eventually killed her.
"I believe he murdered my sister," Figueroa tells RadarOnline.com, in an exclusive interview.
"He abused her for years. She had multiple instances of serious head trauma and it created a brain tumor in her head." She passed away on April 25, 2012.
"My sister was in hell living with him," Figueroa explains. "She was trapped there and there was nothing we could do. She was afraid of him."
"We'd go over to visit and he'd never let us in the house," he says. "When my parents managed to go visit her, she'd make it quick and rush them out because she was afraid he'd come back and beat on her. When she visited our house with him, she'd just stand next to him and be quiet."
"We knew something was wrong with him then, but we could never have imagined something like this would occur. Hearing the news was shocking."
He adds, "But my sister did always used to say that she felt spirits and ghosts in the house. So maybe Ariel was up to something even then."
Grimilda escaped Costa's clutches in 1996, heading to a battered women's shelter. Her children with Castro -- Angie, Emily, Anthony and Rosie -- also escaped.
"If she stayed with Ariel, he would have killed her," her brother says. "She had gone to the hospital and called the police many times but they never did anything."
According to a 2005 filing in an Ohio court, Castro attacked his former wife leaving her with two broken noses, broken ribs, a knocked-out tooth, a blood clot on the brain and two dislocated shoulders.
Her attorney, Robert Ferreri, requested that a judge "keep Castro from threatening to kill Figueroa" and noted that despite having no visitation right, he "frequently abducts daughters and keeps them from mother."
Castro, 52, was arrested on Monday for masterminding the abductions of the three women. His brothers Oneil, 50, and Pedro, 54, were also arrested.
The last time Figueroa saw Castro was at his sister's funeral last year.
"I actually saw him at her funeral and man we wanted to do something to him," Figueroa recalls. "My father had to pull me back and remind me that it was a funeral and it was no place for fighting. But we all didn't like him for what he did to Grimilda."
"Everyone knew he was up to something," he says. "Ariel mistreated my sister for years, and now this."
As for Knight, Berry and DeJesus, the women Castro went on to allegedly kidnap and hold hostage for a decade, "Grimilda never said anything to me about that. That was much later, after she left him. But we always knew he was up to something."
Locked in the basement of Castro's home on Seymour Street for a decade, the women were restrained by chains, neighbor Angel Cordero told RadarOnline.com.
"I broke down the door and I could see a chain," he revealed.
The house of horrors was first exposed when Amanda Berry made a daring escape attempt on Monday night.