Manju, accused Amit's sister, was arrested from her house in Uttam Nagar, a police officer said, adding it was found during investigation she had a hand in the incident.
Amit is already under arrest.
He had allegedly wanted to threaten the woman into accepting his marriage proposal but she refused. Spurned, he allegedly pushed her from the balcony of her house in Avantika Enclave in a fit of rage.
The victim is currently undergoing treatment at Baba Saheb Ambedkar Hospital where her condition is said to be stable, they said.
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Amit has been claiming the woman had borrowed Rs 1 lakh from him and wasn't returning, police said.
He had apparently befriended the woman on Facebook around two years ago and the two got into a relationship, they said.
Amit kept pressurising the woman and her family for marriage but when her parents carried out a background check, they found he was unemployed.
The girl's family had lodged a complaint with the police before the incident alleging that Amit had threatened to create a scene in front of their house if they didn't return the money, he said.
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