Oddly enough, If I think about how best to stage the bodies, I wouldn't place Sheila back in her room as it wouldn't fit well with her having roamed the house, you would have to have her upstairs so that police couldn't see that she was already dead downstairs if they looked in the window (I know I know).
I wouldn't have left Sheila in the boys room either (simply because I think the killer picked them off first). So that narrows it down to an upstairs room for me, why choose the bedroom - I don't know but what is interesting to me is that Sheila's body was 'hidden' on the 'other' side of the bed from the door - anyone looking in the main door would only see June, not Sheila, until they had entered the room properly.