ST. LOUIS—Two young cousins were livestreaming from a St. Louis apartment when one child fatally shot the other before killing herself in what family members say was a tragic accident. Police are still investigating Friday’s incident in which a 12-year-old girl, Paris Harvey, shot a 14-year-old boy, Kuaron Harvey, before shooting herself. Police initially described it as a murder-suicide. The girl’s grandmother, Susan Dyson, told the St. Louis Post-Dispatch that she saw the Instagram Live video the two cousins were making together at a downtown St. Louis apartment the family had rented to celebrate March birthdays. “It wasn’t a situation where they were arguing or anything like that,” said Dyson, who is from the Dallas area. “They were playing with the gun, when they shouldn’t have been. Of course, they shouldn’t have been doing it. I think it just went off. It went off by mistake.” Family members said the …