The suspect, 36, allegedly took the child to a forest, and made her 14-year-old son and his mates, aged 14 and 19, to take turns to rape her.
It is said she then strangled the girl while her son hit her on the head with an axe.
Police say the victim was reported missing from her nearby home on August 24.
Her stepmother was reportedly envious of her and her ex's new wife.
The stepmum and four other people, including the teens and a 28-year-old man, have been arrested in connection with the latest horrific crime to rock India.
"Physical condition and preliminary forensic analysis of dead body substantiated all these facts," said a police statement issued on Tuesday. SHARE THIS STORY USING ANY OF THE BUTTON BELOW ⬇ PLACE YOUR TEXT ADVERT BELOW ⬇⬇⬇