Why couldn't Sheila have fired that second shot ? Why would it have been impossible for one shot to have been followed by a second ?
If Jeremy had been " callous " enough to have left his sister dying,why would it have bothered him any more killing two little boys ? Because you say he's guilty,what you've said doesn't make sense.
How might it have been possible for Sheila, having suffered a shattered spine, have managed to re-position the gun in order to make certain of getting it right second time round? It's perfectly possible that after the first shot the gun would have moved. It's also possible, that after the first injury, she'd have had problems lifting her arms.
He shot the boys DEAD. He didn't leave them to bleed to death.