Kumar's son Rituraj suffered a shoulder dislocation, while nephew Rishabh was also injured in the incident which took place last evening.
Both of them are in their early twenties.
Police have registered an FIR in this connection against the accused under IPC sections of assault and intimidation.
Three persons, identified as Manohar Lal, Ramesh and Rajkumar, have been arrested, a senior police official said.
According to the complaint filed by Rishabh, they had returned from Kumar's South Avenue official residence to South Extension part II where they live with their maternal aunt and sister around 9:40 PM.
An argument ensued between the two parties over the issue during which Manohar Lal and his friends thrashed Rituraj and Rishabh and even misbehaved with women of their family who tried to intervene, he added.
"Someone made a PCR call and the injured were rushed to Safdarjung hospital where they are being treated," the official said.