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A Florida father is nearly out of options in his desperate effort to protect his 8-year-old daughter from her 17-year-old half-brother, who he claims to have witnessed sexually assaulting her — yet who still regularly visits the girl in their mother's home.

"It was the most horrific thing I had ever witnessed in my life," Carl Sumner, of Boca Raton, told FoxNews.com, adding that he sought treatment for post-traumatic stress disorder after the incident. "There's no question as to what happened to my daughter and when it happened. I saw it. I heard her crying."

Sumner, 55, said he was preparing dinner in March 2010 when he heard his then 4-year-old daughter crying inside a bedroom at the family’s home. When he opened the door, Sumner claims, he witnessed the girl’s 12-year-old half-brother sexually assaulting the child, who was naked from the waist down and screaming. His estranged wife, who is the mother of both children and has custody of the girl, does not dispute that sexual abuse occurred.

Sumner intervened and rushed the girl to an emergency room while his wife, Danielle Rowe, called police, who filed a report. The boy, whose name, like that of his half-sister, is being withheld by FoxNews.com because of his age, was not charged but underwent psychiatric evaluation and spent subsequent years in group homes, Sumner said. But despite a court order forbidding the half-siblings from living in the same home, the now 17-year-old, who is Rowe's son by a prior marriage, still regularly visits the Fort Lauderdale home of his mother, who gained permanent custody of her daughter when she and Sumner separated in 2011.

Since they split, Sumner has fought for custody of his daughter and her twin brother, even taking them away to North Carolina against the advice of his attorney, a flight which may have ultimately contributed to the judge's decision to deny him visitation. Sumner says the ordeal has left him financially drained and fearful for his children, who live with Rowe in a suburb of Fort Lauderdale. Rowe acknowledged to FoxNews.com that sexual abuse between her two children did occur, but said her then-12-year-old son did not "penetrate" the girl -- a claim Sumner disputes.

"He [Sumner] wanted my son charged for attempted murder and put on death row," Rowe said. "He wants to destroy a 12-year-old who made a mistake."

Rowe said her family is "trying to heal," and put the episode behind them.

"My daughter is almost 9 years old right now, and she’s a very intelligent girl," Rowe said. "It was my daughter's decision if she wanted to forgive her brother or not, and she chose to forgive her brother. She wants to have him in her life."

Vanessa Prieto, Rowe's attorney, claims the mother of four is following all safety measures to ensure her daughter and son are never unsupervised, saying Rowe is "doing the very best she can" to keep her family together. She declined to answer when asked if the teen is living in the same home as the girl he allegedly molested.

But Sumner said regular visits from the teen violate a court order forbidding the teen and the girl from being together and force his daughter to relive her nightmare. The swimming pool mechanic and former computer support manager opposes any interaction between his 8-year-old daughter and her half-brother – even supervised visits.

"The doctors couldn’t stress enough that [he] will always be grooming her for future attacks. He is at the house all the time," Sumner said of the teen.

"My daughter is reliving the attack every time she sees him. She shouldn’t have to see the person who assaulted her," he said.

Rowe filed for divorce from Sumner in February 2011, though the divorce is still pending. She said she had her son removed from the family’s home and placed into a residential treatment facility – where he stayed for 19 months – immediately following the March 2010 incident. Rowe also claims Sumner refused to participate in family therapy – an allegation he denies.

Prieto said Sumner filed, "numerous false domestic violence claims about my client which have all been dismissed." The attorney also claims Sumner sent "indecent and improper threats" to Rowe and in 2012 took the children into hiding in North Carolina for nine months during which they were absent from school.

Katherine Corrigan, Sumner's former attorney, said, "Carl was in absolute panic" when he took the children to another state.

"His concerns are 100 percent valid with respect to not just one child, but both of his children, and they shouldn’t be minimized," she told FoxNews.com. "I believe that those kids are still in danger."

"Danielle is in many respects naive about her son," added Corrigan, though she said, "She's in a difficult spot. They are all her children."

"She’s the one who didn’t want to work on a resolution in my opinion," she said of Rowe.

A Florida family court judge granted Rowe full custody of the children in May 2012. Rowe claims she allowed Sumner to continue to see the twins – first with supervised visits and then unsupervised ones beginning in June 2013.

But last April, Broward County Judge Dale Cohen of the 17th Judicial Circuit Court in Florida denied the father any visitation rights, Sumner said. A court-appointed guardian testified that Sumner was "a danger, possibly physically and definitely mentally, and a flight risk," according to Prieto.

Rowe said her estranged husband took their kids and grilled the girl about what her older half-brother had done. But Sumner is simply desperate to protect his only two children – even if it meant defying the court, said Corrigan.

"It’s very sad," she said. "He’s a good dad. He absolutely loves his kids and will do anything to protect them."