We will never know the order of the shots, but I think the pattern of the shots suggest the shots to the boys were all done at the same time. Sheila then reloading, this may have been while Nevil was having his shower? A .22 rifle is lethal at close range JB would only have used two or three shots not nearly a whole magazine.
You still need to explain the third or fourth shot to Nevil which hit his shoulder, I can only think this occurred as he came up the stairs the shooter shooting downwards etc.