Shakeela Naz, 45, and her 22-year-old son Shazad, from Derby, said Rukhsana Naz had brought shame on their family by refusing to have an abortion after becoming pregnant by her lover while her husband was in Pakistan.
Mrs Naz, a Muslim, helped her eldest son strangle her daughter using a plastic flex in March last year. They then placed her body in a homemade sack and dumped her in a farmer's field 100 miles away.
Mr Justice Tucker said: "It was a particularly horrific offence, involving as it did the death of a young pregnant woman, who was already the mother of two children, at the hands of her own family."
Mrs Naz, a seamstress, and Shazad had admitted being present at the time of Rukhsana's death but denied murdering her and her unborn child. Iftikhar Naz, 18, Rukhsana's younger brother, who admitted helping to dump the body, was acquitted.
James Hunt QC, for the prosecution, told Nottingham Crown Court that Rukhsana was murdered because she had disgraced the family.
The court heard that she had been married by arrangement at the age of 15 to a man in Pakistan. She visited him twice, on each occasion to conceive a child. But she was secretly carrying on a love affair with Imran Najib, a childhood sweetheart, who was also tied to an arranged marriage. The couple kept in touch by pager; police were only able to establish Rukhsana's identity by tracing back the pager number that was written on her hand.
The court heard that during police interviews Mrs Naz said: "We didn't want to kill her - it was written in her fate."
Three weeks before the murder, Mrs Naz confronted her daughter, kicking her in the stomach and demanding that she have an abortion. She suggested she should take a paracetamol overdose to induce a miscarriage but when Rukhsana refused Mrs Naz called in Shazad, then the head of the family, to "talk sense" to her daughter.
When she defied their orders, Shazad strangled his sister in a fit of rage. Mrs Naz held her legs while he tightened a plastic flex around her neck for three to four minutes.
Her younger brother Iftikhar watched helplessly. He told the court his brother had threatened him with a knife and forced him to help with the disposal of the body. "Shazad had a scary look on his face, an evil type of look. He did not seem scared at all," he said.
"Shazad said we should bury the body and I said I wouldn't. He said, `Look where you are.' I took that to mean that we were in a field late at night and that he could kill me too."
The week before Rukhsana died, the court heard that Mrs Naz and her son insisted that she draw up a will to give them legal guardianship of her two other children in the event of her death.
Mr Hunt said: "She was then asked to empty a trunk that was in her mother's house. She was being asked to empty the container which her own body was going to be placed into."Reuse content