Am I right in thinking there was a connecting door from the master bedroom through to where the twins were sleeping and this is where Sheila may have been heading before she was shot?
If she were in the hall the easiest way to their room was to walk down a couple of stairs through the hall to their door.
If he were blocking the hall maybe she was trying to run to the other side to try to escape through that door.
He could have placed her there to make it look like she had just come from killing the boys. We don't know what he had intended since he hasn't told anyone not even Julie.