Alesha Ahmed was arrested after the armed raid at her parents' house in Warrington in August 2010.
The 22-year-old law student was questioned on suspicion of organising the robbery, in which her mother Farzana and three siblings were tied up and threatened.
But it was during questioning that she told police she had witnessed her parents murder her elder sister Shafilea in the house almost a decade earlier.
The 17-year-old 'A' level student was suffocated in front of Alesha and the other children, and her body dumped in the Lake District, after refusing to submit to an arranged marriage.
Alesha's decision to give evidence against Iftikhar and Farzana ultimately led to their conviction for the murder and they were both sentenced to 25 years each in August.
Alesha confessed to organising the robbery but her sentencing was delayed until the completion of her parents' trial.
The court heard how the raid involved three masked men who burst into the family home brandishing a pistol, a hammer and a metal bar. The men, who subjected the family to a terrifying ordeal have never been caught.
Sentencing her to 12 months imprisonment, suspended for two years, at Southwark Crown Court, Mr Justice Irwin said it was an "extraordinary" case adding: "In my view this is a case for mercy."
He told her: "You were forced to carry this darkest of secrets as your parents sought to evade justice."
Outlining the prosecution case, Owen Edwards said: "Her arrest was the catalyst for her decision to tell the police what they had long suspected.
"Alesha Ahmed provided a witness statement saying she had a witnessed her sister being killed by her parents.
"Without a shadow of a doubt her participation in the trial of her parents in which she gave evidence over eight days during May 2012 was the key feature in their conviction and also the remedying a near decade of injustice."
Isabella Forshall QC, defending said it was a "unique" and "completely extraordinary case".
In mitigation, Miss Forshall said Alesha had suffered immensely after witnessing her sister's murder.
She said: "Her judgement was clouded by severe post-traumatic mental illness."
She added: "This particular house was the killing ground of Shafilea and its privacy was a scene and instrument of brutal repression of Shafilea and the other children."
Alesha will remain in the witness protection programme and Mr Justice Irwin said: "Despite your obvious intelligence and despite the help you may be given, your future will be overshadowed by all that has happened and it may be a long time before you achieve peace of mind and anything like normality."