Shafilea Ahmed drank bleach in Pakistan after her mother told her she would not be going home, her sister has told a court.
Alesha Ahmed was giving evidence in the trial of parents Iftikhar and Farzana, accused at Chester Crown Court of killing 17-year-old Shafilea at the family home in September 2003.
Alesha, then 15, witnessed the murder but harboured the secret for seven years.
She told Chester Crown Court that her sister drank the poisonous liquid while on a family trip to visit relatives.
After drinking the dose, her throat was so badly damaged that she could not eat properly for months and her weight plunged to five stone.
"There was a general conversation and a remark was made to Shafilea which I think triggered her drinking the bleach," the witness said.
"(My mum) said something along the lines of Shafilea will be staying there and not going back. It was done in a humorous way but obviously it was very serious as other people were present."
Andrew Edis QC, prosecuting, told the court on Wednesday that Alesha later witnessed "an act of suffocation" on Shafilea by both her parents.
He said they put their hands over Shafilea's face "to close her airways so she could not breathe", adding: "She had a bag forced into her mouth."
The teenager's decomposed remains were discovered in Cumbria in February 2004 but it was not until 2010 that Alesha provided the "final piece of the puzzle" about her death, Mr Edis said.
The couple allegedly murdered their "westernised" daughter because they believed her conduct was bringing shame on the family.