I'm taking that second shot as being purely " accidental "and not by Sheilas' hand.
The first shot was an attempt at suicide,,,which after a short while,would have eventually killed the girl,,but Sheila did not administer that second shot.No way.
As I've already surmised,that after the first shot,Sheila put the rifle on the windowsill upstairs,then lay on her bed.
It was after that she was moved,then the accident happened when the rifle went off " unexpectedly " during the investigation,,,then the big cover-up. Nobody likes to admit that they shot someone accidentally.