A California mother is under arrest after one of her sons shot and killed his twin brother. Neighbors of the San Bernardino family were stunned to hear the news. Police reportedly said the kids were home alone when they found the unsecured weapon in the master bedroom. They allegedly loaded it themselves with ammunition they found nearby. Investigators say the gun accidentally went off while one twin was pretending to shoot the other. InsideEdition.com's Mara Montalbano has more.