Peter Booth a witness in the trial of David Dearlove at Teesside Crown Court who is accused of killing 19-month-old Paul Booth in October 1968.
Mr Dearlove smashing the toddler's head against a fireplace.
Peter was not questioned by police at the time but - after several attempts to have the case reopened - he was interviewed in 2015, the court heard.
Prosecutor Richard Wright QC said Peter told officers he crept downstairs to get a drink of water on the night of his brother's death at home in Haverton Hill, Teesside.
The alleged crime went unreported until Paul's brother Peter Booth, who was just four when Paul died, saw a photograph of his brother, being held by Dearlove on Facebook two years ago. He had not been asked to give any account in 1968.
The prosecutor said that Peter Booth contacted David Dearlove Jr to demand he remove the photograph from Facebook and made a decision to contact police.
"Through a gap in the door into the sitting room he had seen David Dearlove swinging Paul Booth around whilst holding on to his ankles and had watched as his stepfather smashed the little boy's head into the fire surround, causing the fatal injury to his skull by the impact".
"The death of Paul Booth had been no accident, it had been as the result of a deliberate act".
Carol, who is now dead, had three children at the time and the jury was told Dearlove allegedly abused the children which accounts for the cruelty charges.
Stephanie also claimed that Dearlove would often lie on top of her in bed with his full weight and then slap her if she cried.