The father of two children who were seen being abused in a video said he was in tears after viewing the footage.
The four women laughed and swore as the abuse continued
Zara Olver, 21, her mother Carole, 48, and her two sisters Serenza, 29, and Danielle, 19, admitted child cruelty charges at Plymouth Crown Court.
They were heard laughing and swearing as they encouraged the tearful children to punch and kick each other.
The father, who had been serving with the Army in Iraq, returned on leave to find the images on a digital camcorder.
The case, which is thought to be the first of its type, came to court after the father passed the video to social services who alerted child protection officers at Devon and Cornwall Police.
In the video, the father saw his son crying and wearing just a nappy and T-shirt, being goaded into punching his sister after she hit him and being called a "wimp" and "faggot".
David Gittins, prosecuting, said: "He found the scenes so shocking he was deeply disturbed... he was reduced to tears.
"The film showed the children being encouraged to progress to higher levels of violence.
"When the boy falls down he is told to get up and not to be a wimp.
Shouting and laughing
"The girl tries to leave the room but is told to hit her brother and told to boot him and one of the women said, 'Don't let a boy win'."
The girl was shoved back into the fight when she tried to get away, said Mr Gittins.
"She can be seen screaming. Someone threw in hairbrush to encourage the boy to pick it up to hit his sister and towards the end of the film the defendants can all be heard shouting and laughing.
"All the adults present appeared to find it funny and this can only be described as sick," he said.
Investigating officer, Det Sgt Barry Walters, read a statement from the family which said: "Our priority has understandably been the welfare of the children and to isolate them from the details of the proceedings and protect them from any further mental and physical stress.
"The gravity of events has placed an enormous strain on our family, who had no idea that these women were capable of acts of such wanton cruelty.
"We are now very pleased these events are nearing a conclusion and as a family we can move on and ensure the remainder of their childhood is safe and happy."