You don't have to believe what she said if you don't want to, but that is what she said. I believe the jury were warned as to her credibility issues in the summing up. What "antis"? Personally I find the idea of a third party a complete non starter.
There are as many theories about how Sheila died as there are posters, and they will only ever be theories (unless JB comes clean). Personally I think she was asleep in bed, that she possibly (hopefully) never knew her boys had been killed and wandered out of her bedroom when she heard the commotion downstairs. She went into her parents bedroom to find her father, only to find her mother either wounded or already dead. The next thing she knew JB was there in the doorway pointing the rifle at her and she moved around the bed towards the connecting door to the twins room. He caught up with her and forced her at gun point to sit down, and he then delivered the first shot. Although it didn't kill her it caused her to slump onto her right side with her head against the cabinet making the second shot awkward. He then pulled her by her legs to straighten out her body before delivering the second shot under the chin. Or something 
There must have been a lot of noise in that house that night. Not only the shots, but screams and moaning (hard to write this), dogs barking. Why would she be awoken by the commotion downstairs, when there had been so much going on upsstairs right beside her bedroom?
Did she hear all this horror, and then partially make her bed before stepping out of her room, because that bed does not quite look like someone slept in it, rather it looks like someone had lied on top of it, but not under the covers.
How could Jeremy count on Sheila sleeping through all this - that seems a little bold??