June was a light sleeper. In fact she was a bad sleeper who suffered from nightmares which kept her awake. Latterly,for whatever reason,she'd been seeing her GP regularly for 6 months up to the tragedy,probably stress because of worry over Sheila. June would have heard a pin drop let alone the sound of a gun going off.
Neville would have initially been downstairs after returning late from walking the dogs,then showering and very likely heard Sheila ranting,upstairs. He may have heard gunshots but perhaps thought that the shots were fired upwards as a " warning " and not the fact that Sheila could possibly have shot the twins at that point. On hearing shots,Neville would have rang Jeremy, then he would have made his way upstairs to see what was going on and while upstairs was shot himself so he retreated back down again into the kitchen where Sheila had followed,most likely still ranting and revealing the very sick side to her illness.
( I was witness last year to a very sick woman whose shouting sounded like that of a man and a neighbour had reported the shouting as being a " drunken man " causing a breach of the peace,but in actual fact the woman suffered from paranoia and spent 3+ months in hospital There'd always been the shouting,but it got progressively worse. The change in the tone of her voice was creepy to say the least. )
Their moods can change in an instant and their main attackers are those that they love,strange as it may seem.