My view, rightly or wrongly is that Sheila after she had shot the rest of the family prepared herself to die. Firstly she would take off all her bloodstained clothing and put them in soak. Then she would almost ceremoniously take a shower or bath. Then putting on a clean night dress she would proceed to paint all her nails, as was her custom apparently. She kept her nails meticulously clean and painted bright red. After all this preparation she would proceed to take the rifle and pointing it at her throat so-as not to spoil her looks, turn her head away and took her first shot.
Finding she was still alive but bleeding profusely panicked and held her hand to her throat thus staunching the flow of blood. Then wiping her hand on her night dress had another go. This time holding the end of the barrel of the rifle in order to steady her hand placed the muzzle against her throat again, this shot was unfortunately successful in her purpose. For how could she live with such a crime on her conscience?