Investigators are at the Montebella Apartments in the 4000 block of W. 34th Street, where police said the two 9-year-olds were toying around with the weapon when it went off.
The shooting happened just after 3:30 p.m. on Wednesday.
Police rushed to the apartment complex and found the boy with a gunshot wound. He was taken to Memorial Hermann Greater Heights Hospital, but died from his injuries.
Now investigators are trying to figure out who owns the gun, and how the boy's cousin got his hands on the weapon.
The Harris County District Attorney's office will decide whether a parent should be charged.
Children under the age of 10 cannot be held criminally responsible, police tell ABC13 Eyewitness News.