Mr Usuga - who leads the Urabenos criminal gang - is still at large.
His cousin, Arley Usuga Torres, known as Zero Seven or Cop Killer, was arrested at a farm in north-western Colombia.
He is accused of killing more than 20 police officers and is wanted in the United States for drug trafficking.
At least eight other people were arrested at the operation in Uraba region.
Weapons and more than $3m (£1.8m) were seized in the raid, officials said.
In a separate operation in the city of Medellin, Colombian police arrested Dario Usuga's sister, Nini Johana Usuga.
She is accused of drug trafficking and making illegal weapons.
Colombian police say they are closing in on Mr Usuga, the country's most wanted criminal.
In November, another alleged senior member of the gang was arrested in Spain.
The Urabenos was partly formed from members of paramilitary groups established in the 1980s to tackle left-wing guerrillas during Colombia's long civil war.
They are linked to a string of murders and kidnappings, as well as extortion and drug-trafficking activities.
The organisation take its name from the region Uraba, which borders Panama.
The area is in a strategic position for smuggling routes towards Central America and the United States.
Colombian police have arrested two relatives and several alleged associates of a prominent gang leader, Dario Antonio Usuga - known as Otoniel.