Satpal Singh and his associates Kanwar Neeraj, Dinesh Kumar, Amit Gupta, Punit and Ankit Sharma were arrested on Saturday, three days after Ashok Kumar was shot at in Sagarpur in south-west Delhi.
Kumar had succumbed to his injuries later at a hospital.
During investigation, police found that Kumar had married Singh's daughter against his wish but the two families patched up after the couple had a baby girl.
Contradictory statements by the family members about who was last seen with Kumar led the police to suspect Singh's role. Police also got to know that even after the patch up, both families had strained relations.
On Saturday, when Singh and the five others gathered in Sagarpur, police rounded them up and arrested all of them.
"After their marriage, the girl's family had accepted everything and a baby girl was also born from the wedlock of Ashok and their daughter. But Ashok continued to taunt them over the same issue, thus they decided to eliminate him," a senior police official said.