Diane Jones, 21, was trapped in a bedroom with her daughters, Shauna, 2, and 13-month-old Sarah Jane, when their house was set ablaze by petrol poured through the front door.
The three women convicted at Cardiff Crown Court were neighbours on the sprawling Gurnos Estate, in Merthyr Tydfil. The court was told that they carried out a revenge attack on Miss Jones, who lived with Shaun Hibberd, who was in prison at the time.
Donna Clarke, 27, who was jailed for 20 years for arson, had been spurned by Mr Hibberd, who returned to Miss Jones after an affair with her. Weeks before the fire in October 1995, Clarke told a friend: "I can't wait to get my hands on Diane - I'm going to petrol-bomb her."
Her aunt, Annette Hewins, 31, who supplied the petrol, was sentenced to 13 years for arson. Both she and Clarke were cleared of murder. Denise Sullivan, 25, another former lover of Mr Hibberd, was convicted of perverting the course of justice and given four years.
The guilty verdicts were delivered after a four-month trial and caused an uproar in the courtroom. The three defendants, white-faced and shaking in the dock, shouted their innocence as the public gallery, packed with Miss Jones's relatives, erupted into cheers.
The attack was carried out at 2am as Miss Jones and her children were sleeping. Clarke, a mother-of-two, ignited petrol which she poured into the hallway. She left the scene, but soon afterwards was seen nearby, watching firefighters tackle the blaze.
Neighbours who tried to rescue the family were driven back by intense heat and smoke. When fire crews found Miss Jones's burnt body, her arms were stretched around Sarah Jane, as if trying to shield her. Shauna was lying feet away.
The jury was told that Miss Jones shouted out of the bedroom window for help, screaming: "My babies! My babies!" But flames in the hallway were sucked upstairs by the rush of air from the open window.
South Wales detectives uncovered a long-standing feud between the women.
Miss Jones had threatened to reveal Clarke's affair with Mr Hibberd to her partner, Simon Owens, who was shortly to be released from prison after serving a two-year sentence for manslaughter.
Despite both having slept with Mr Hibberd, Clarke and Sullivan were such close friends that they nicknamed themselves Cagney and Lacey after the two women detectives in the American television series.
Mr Hibberd, who attended the funerals of his partner and children handcuffed to a prison officer, told the court that Clarke would not leave him alone after the affair.
She pestered him with telephone calls and took to dressing like Miss Jones, he said, watching to see what she was wearing in the morning and then going home to change into similar clothes.