It doesn't say anything about making an episode where Sheila is the killer and I don't think that is the intention.
'Each of the six episodes of The Murder At White House Farm will look at the case from the viewpoint of a different character — including Bamber himself, his parents, his sister, and her former husband.'
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The viewpoint of Sheila will suggest she had the mindset to kill.
They are not going to make a 6 part drama without giving the public something to debate - a 'who done it'. It creates more viewers.
The least they have to do is show how & why the police believed Sheila was the killer.This together with Julie, the relatives & the silencer will get people who know nothing about the case logging onto the OS etc. As you did after watching the much shorter & more mild CTSB.