Sheila was able to kill everyone (Ralph, june, Daniel and Nicholas) because her parents were isolated from one another when she was overcome by the devil, and shot everyone...
Kids were dispatched in their bedroom, with no evidence to suggest that either Ralph, or June, or anyone else for that matter, had stepped in to try and prevent the kids from being shot, or that anyone had attempted to keep either child alive once both had been shot? With this in mind then, we can safely assume that if Sheila was/is the killer, that she was able to dispatch her boys undisturbed by interference from any living sole...
This suggests to me that Sheila might have either shot and killed her boys first, by quickly discharging 8 bullets into the heads of her children, and that they were killed in the blink of an eye before either grandparent could possibly do anything to prevent it occurring, or Sheila shot and killed her boys later once either Ralph and june had been shot but not necessarily killed, but let's just say incapacitated, or isolated from each other?
One thing we can be sure about, which could prove to be very telling, is that both of them were shot whilst sleeping in their respective beds, oblivious of the identity of the person who had cut short their young lives, unlike either grandparent who must have stared at some point directly Into the killers eyes?