Deep Singh Malviya (32), his wife Rukmani (30), daughter Sanjana (5) and son Sourabh (3) along with one Karim Khan were found strangulated inside the car.
Based on forensic evidence, police zeroed in on Deep Malviya's brothers -- Vinod Singh and Bane Singh -- and his nephew for the murders, police said.
The three have allegedly confessed during interrogation to murdering the victims over a family property dispute.
Additional Director General of Police (ADGP) Vijay Yadav has confirmed the arrests.
Deep Singh, who hailed from Khedla village in the district, had been engaged in real estate business and had a criminal background, police said.