It was bound to happen; when you add alcohol and guns together the outcome is not always good. They were at a party and then came home and keep on drinking. A long night of drinking didn't help and out came the gun.
While posing for picture she did the unthinkable and shot her brother in the head. Either no one checked the gun or didn't care because the rules of safe gun handling where totally ignored. Even though it was an accident he might be charged with manslaughter.
The details are going to make the difference if she gets charged or not, whose gun was it and how it was handled will play a big part in the decision but never the less she will have to live with it for the rest of her life