Shahena Uddin, 19,  was subjected to a "regime of fear" at the hands of Salma Begum, St Albans Crown Court heard.
Ambulance staff called to the family home in Leavesden Road, Watford, on 11 October last year found her body in the bathroom.
Mr Justice Spencer adjourned sentence until Monday.
But he told the jury Begum would receive a mandatory life sentence and he would have to decide the minimum term she should serve.
The jury cleared Ms Uddin's oldest brother Suhail Uddin, 35, of murder and an alternative charge of manslaughter.
He and three other brothers, Jewell, 27, Jhuhal, 33 and Tohel Uddin, 24, along with Ms Uddin's older sister Rehena Uddin, 22, were found guilty of causing or allowing the death of a vulnerable adult.
All five and Begum were found guilty of conspiracy to pervert the course of public justice by attempting to impede the police investigation.
At the end of the prosecution case, Mr Justice Spencer ruled that Laila Begum, 25, the wife of Jhuhal Uddin, had no case to answer on charges of causing or allowing the death of a vulnerable adult and conspiracy to pervert the course of justice.
The prosecution said Ms Uddin was beaten to death at the family house during the night of 10-11 October.
Paramedics found Ms Uddin's body, bearing the marks of a severe beating, at her home where she was frequently beaten.
Pathologist Dr Robert Chapman, who carried out a post-mortem examination, said her head was covered in bruising, her lips cut and her eyes black and swollen.
He found bruising to her shoulders and legs and strike marks across her right breast, and it appeared a "rod-like" object had been used to strike her.
He concluded she choked on her own vomit after becoming unconscious.
Begum, the only defendant to give evidence in court, claimed Ms Uddin had died because she was sick and had hit her head on a toilet.
The sister-in-law of a teenage girl who was beaten to death in the family home collapsed in the dock when a jury convicted her of murder.