The police were called out to a fourth-floor family apartment in the town just after midnight last night, newspaper Le Parisien reports.
A 17-year-old boy was found shot in the living room, but his brother was missing.
It is thought the boys stayed up late after the rest of their family had gone to bed, and were playing with a 9mm handgun.
One of the boys accidentally fired the gun at his brother, then fled the scene.
Police found him at the bottom of the building, disorientated and asking after his brother. He and a third brother were taken into custody for questioning.