Zainab Al-Hilli's state of health is understood to have improved in recent days, allowing her to travel back to the UK.
Zainab left France this morning at around 8am. Police from Britain accompanied the little girl.
Her departure follows that of her younger sister, Zeena, who returned to the UK last week after escaping the brutal attack unscathed. The four-year-old survived by cowering under her mother's skirts but she was only discovered eight hours after the killings.
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"All it takes is for somebody to see me - and I could just die, just like that. I could want to go to the shop and not come home."
Before the killing, Peters searched the internet for "serial killers", "treatment of child killers in prison" and "premeditated murder".
Arriva Rail North, Greater Anglia and South Western Railway are all affected.