The Old Bailey court in London sentenced Sabrina Kouider, who is undergoing psychiatric treatment, and Ouissem Medouni for the September 2017 killing of 21-year-old Sophie Lionnet.
“She died as a result of purposeful and sustained violence, and not by accident,” state prosecutor Aisling Hosein said when they were convicted last month.
“They were both jointly involved and came up with a plan to try and destroy her body and escape responsibility for this horrendous crime,” she added.
The jurors deliberated for a week before unanimously convicting Kouider and ruling by a majority decision of 10 to 2 on May 24 that her partner Medouni was guilty as well.
Both had denied murdering Lionnet, from Troyes in eastern France, although they had admitted during the two-month trial to burning her body.
Murder victim Sophie Lionnet. Photo: AFP
The court heard how the couple had interrogated and tortured Lionnet over their belief she was conspiring with one of Kouider's former boyfriends – Mark Walton, a former member of Irish band Boyzone – who they claimed sexually abused members of their family.
Kouider, who has two children, told London's Old Bailey court that she hit the au pair “really bad” with an electrical cable.
The victim's mother called the guilty pair “monsters” in a statement read to court earlier.
Catherine Devallonne said she “fell into shock and was hospitalised” after police broke the news that her daughter, whom she described as a “reserved young girl”, had been killed.
“I've been living this nightmare ever since,” she added, explaining that the victim's brother was suffering from “mental trauma”, and had been placed with a foster family.
“Those monsters beat her to death,” she said. “They left her hungry. They took away her dignity and eventually her life”.
Catherine Devallonne, the victim's mother, arrives at court in London in April for the trial of the accused couple. Photo: AFP
Police described how the victim was subjected to a “series of 'interrogations'…over a 12-day period, in a bid to force Sophie to confess to various false crimes they had accused her of.”
Kouider filmed some of the sessions, which they planned to hand them over to police as evidence of the nanny's guilt.
“We will never know the full extent of the horrors Sophie had to endure,” said Scotland Yard detective Domenica Catino.
“Even in death, the torture, abuse and humiliation continued by placing her partially-clothed body into a suitcase with no regard for even a semblance of a burial.
“It was clear that together the couple made the decision to torture Sophie and then in a cowardly fashion blamed each other for her death,” added the detective.
Firefighters discovered Medouni trying to burn Lionnet's body on September 20 at the couple's home in Southfields, south west London.
Neighbours alerted the authorities after noticing smoke and a “horrible” smell coming from the property.
Firefighter Thomas Hunt told the court that he confronted Medouni after he found human fingers and a nose as he put out the fire.