Its common sense that if attempting to disarm someone you run a high risk of getting shot yourself.
Nevill would not be deterred by Sheila holding a gun for shooting rabbits. He would have no hesitation in getting the rifle back. He had no choice. As head of the family he had to protect himself and the other 4 people in the house.
He would disarm Sheila when close enough. Or use an object to charge her. Either way very unlikely to get shot.
Nevill would then lock the rifle away, make sure Sheila was calm and join an asleep June in bed. Him & Bamber had an early start.