Bruce Leonard, the father of the boy beaten to death by a church counselling session, has testified in court against his own daughter.

He described in vivid detail how his two sons were whipped because they had been suspected of sexually molesting their sister and their half-sister's children. "Blows that hurt drive off evil", said Leonard, according to Syracuse.com, apparently quoting Proverbs 20.30. One son, Lucas, 19, died of his wounds and the other, 17-year-old Christopher was seriously injured.

The surviving son has admitted the pair did molest their sister Grace and their half-sister Sarah Ferguson's children.

Leonard was originally going to face charges alongside Ferguson, who is his daughter, but accepted a plea bargain just before the trial and then testified against her. Leonard pleaded guilty to first and second-degree assault in return for a 15 to 16 year prison sentence.

Ferguson faces charges of second-degree murder, first- and second-degree manslaughter, two counts of first-degree assault and two counts of first-degree gang assault.

Leonard described how their pastor at Word of Life church Tiffanie Irwin had organised the counselling session. He said the beatings began with a punch and a slap and then progressed to whipping with an electrical cord.

They were beaten "so [they] would understand the hurt that they caused," he said.

Leonard said he had handed the cord to Ferguson who started to beat them around their groins. Both boys fell twice during the beating but were forced back up so it could continue, he said. At no point did anyone suggest they stop, said Leonard.

He estimated the two boys were whipped 20 times each. The assault only stopped when they noticed Lucas' penis was bleeding.

He said the next he noticed Lucas was not breathing and was rushed to hospital.

Christopher Leonard has admitted earlier this week that he inappropriately touched Ferguson's children but did not sexually abuse them or commit a sex act with them.

Grace Leonard, his sister, said her brothers had touched her inappropriately from the age of 4 or 5 years old until she was 13.

The case continues.