Broadly speaking that is how I see it too. We can build a picture from the scene how it went down. Your conclusions regarding Nevill make sense. Also with June evidence from the scene suggests that she staggered around the bed after the initial attack being finished off between the eyes.
When it comes to Sheila, Jeremy may have just pounced on her wherever he found her by the time he ran back upstairs following the unplanned detour by Nevill. As long as she wasn't in her bed. As it happened, Sheila was out of bed by this time and tending to a severely injured June. He barked at her to get the f*** down now, which the poor woman would have no choice but to have complied with before being shot the first time. Jeremy finishes off June between the eyes. Then comes the moment Jeremy turns white, when he realised Sheila needed a second shot to finish her off.
He positions the gun on her, making sure to get her print on there (just the 1 print mind) smugly plants the bible to shore up the deranged nature of his sisters actions, to leave no doubt, or so he thought. Then he goes about setting the scene, firing more shots where necessary.