Porter, of Pittsburgh, was parked in a Steak 'n Shake parking lot in suburban Munhall when he was shot. He died at a hospital Thursday.
Allegheny County homicide detectives believe Porter planned to sell heroin to Young and another still-unidentified suspect when he was, instead, shot in the head.
Khalifa has tweeted that his "uncle was younger and more level-headed than all of us."
Young denied involvement to reporters as he was taken into custody.
