Martin Thornton said that he did not hear any argument before his stepmother stabbed his father with a kitchen knife in the living room of their home in Atherstone, Warwickshire.
He told Oxford Crown Court: "I heard my father scream so I came downstairs and I got halfway down and Sara was standing at the bottom and she looked at me and just said, 'Martin, I have killed your father'."
Mr Thornton was giving evidence on the second day of the retrial of his stepmother which was ordered by the Court of Appeal. Mrs Thornton, 41, denies murdering her alcoholic husband in 1989. She says that she stabbed him accidentally after a row and that he regularly beat her up.
At the time of the killing Martin Thornton, now 27, was living with his father and stepmother while he helped the former run a shop in Atherstone.
Mr Thornton said yesterday that two days before the stabbing his stepmother threatened his father, who was drunk, with a knife during a row and warned him not to touch her daughter Luise. He told the court: "She said, 'if you touch my daughter I am going to kill you'. I then disarmed Sara and put the knife into the kitchen sink.
"I would say that she was very angry and that it was threatening but I don't think there would have been any further action taken at the time."
Describing the night his father was stabbed Mr Thornton said: "He was drunk. There was an argument between my father and Sara ... my father did say that he wanted her out of the house and he did throw her clothes out of the bedroom window into the front garden."
That evening, Luise, Mrs Thornton's daughter from a previous marriage, was sent to stay with friends and Martin Thornton went out for a drink with his stepmother. They left separately to avoid giving the impression that he was taking sides and he returned home before she did.
He said: "My father was lying on the couch fast asleep. I asked him if he wanted a cup of tea and there was no response so I left him asleep on the couch and went to bed.
"Sometime after that I heard the front door open. I automatically assumed that this was Sara returning home... She came up and opened my bedroom door and looked in. I did not acknowledge that she was there." He said that he then heard somebody rummaging around in the cutlery drawer in the kitchen and then everything was quiet for a couple of minutes before he heard his father scream.
He added that when he went downstairs: "Sara said as I went into the lounge not to pull the knife out of him but it was already out and was lying between his legs on the floor."
The case continues.