Jairo Saenz pleaded guilty to racketeering and weapons charges six months after his brother, Alexi, pleaded guilty to eight murders.
"I knew what I was doing, and I knew I was wrong," Saenz told the judge. "I came to the U.S., and worked for two years. Then, I went to the streets, and joined the MS-13 gang."
Saenz said he acted as the driver to many of the shootings.
He faces 40 to 60 years behind bars when sentenced on June 13.
The bodies of Cuevas and Mickens were found bludgeoned to death in September 2016. Prosecutors said they had been killed following a dispute at their high school.