Sumit Kumar with his two friends Pradeep Kumar and Dharamveer on September 19 fired at his sister (16) and mother (38) after strangulating them in Manesar area, they said.
Two country-made pistols and cartridges were seized from the car used in the commission of crime, police said.
Sumit had objected to his mother's alleged prostitution business which she was operating from her house in Nagafgarh, they said.
"Sumit along with his friends hatched a plan to eliminate his sister and mother after he came to know his sister was also involved in prostitution," Deputy Commissioner of Police (West) Sumit Kuhar said.
He persuaded his mother and sister to visit a religious place in Rajasthan and took them to Manesar along with his two friends on September 18. On the night of crime at 11.30 PM they reached Manesar near auto stand in a car and shot his mother and sister at point-blank range, they said.
They were arrested yesterday after a tip-off was received by Palam Vihar Crime Branch unit that some suspected youths involved in crime were coming at Manesar on Delhi-Jaipur National Highway 8, they added.
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