Mahila Court Judge M P Subramaniam also sentenced the accused Manoharan to three life sentences for various crimes, including conspiracy, rape and murder.
He also handed down three years rigorous imprisonment to Manoharan for abduction.
In October 2012, Manoharan, along with a friend Mohanakrishnan who was key accused in the case, had abducted the siblings and killed them after raping the girl, prosecution sources said.
The duo raped and murdered the girl and also her brother as he was a witness to the crime. They later threw the bodies in a canal near Pollachi.
Mohanakrishnan, who was arrested on the day of crime, was shot dead by police on November 8, 2010 in a vehicle while he was being taken to the scene of crime after he allegedly tried to snatch the pistol of a policeman in a bid to escape.
On October 29, the judge had convicted Manoharan under sections of IPC 20 B (conspiracy), 364 A (abduction for ransom), 376 (2) (f) (g)(gang rape and rape of a girl less than 12-years-old), 302 (murder) and 201 (concealment of evidence) and fixed today for pronouncement of the sentence.