For me, you have to imagine some unlikely events to conjure up a scenario for either of them to be the killer.
I've always thought that JB intended to shoot them all in their beds, leave the rifle on Sheila and voila but NB woke up and pushed past him as he opened fire in the bedroom. Perhaps Sheila was woken up by the commotion downstairs, went to investigate, saw her mother and was still dazed, wondering WTF was happening when JB came back up the stairs with a reloaded magazine.
JB was wearing a mask and simply stuck the gun under her chin, ordered a terrified Sheila to lie down and pulled the trigger. Horrified he has to pull the trigger again, he stages the scene, puts the open bible on her but it slides off, closing itself and producing the mirrored blood the court of appeal pointed out, he opens it again, perhaps it falls open to the same well read page and he takes off the silencer and leaves, flicking the V sign at his hated mother on the way out.