The Chester Crown Court found that Iftikhar and Farzana Ahmed, both originally from Pakistan, suffocated their 17-year-old daughter, Shafilea, in 2003.
During the trial in northern England, Shafilea's sister Alesha told the jury that her parents pushed Shafilea onto the couch and she heard her mother say "just finish it here" as they forced a plastic bag into the girl's mouth.
She said her parents thought that Shafilea was too westernised. She also said Alesha refused an arranged marriage.
Police investigated hundreds of cases of forced marriages last year.
The highest incidence of reported forced marriages is in Muslim communities. Britain is home to more than 1.8 million Muslims.