Once everyone was dead Bamber just had to get Sheila out of bed and to where most of the shooting took place. Which was only a few feet from where she was sleeping.
He could put the rifle down, in the main bedroom. Then open Sheila's bedroom door.
He could attempt to persuade her to come with him to the main bedroom. Being Sheila's brother there is no reason why Sheila would resist. If she did see June lying down in the main bedroom and started to resist, Bamber could use his superior power to restrain her and shoot her.
Or Bamber could just wait until a half asleep Sheila was standing up, then carry her a few feet. Which would take less than ten seconds.
Or as Scipio suggested , Sheila was woken and lead at gunpoint.