Jeremy Bamber is in prison because he would have inherited. Once his mam and dad were killed, there was nobody to protect him from Robert Boutflour and his daughter. His daughter had already labelled Jeremy as a 'threat' (to their livelihoods) prior to the killings. This is a very telling event (IMO) - as it clearly marks Jeremy's card. Some of Jeremy's own behaviour has simply reinforced the relatives' suspicion of him. The result being - anything that could be gleaned from events that pointed towards Sheila was dismissed. Everything that could be interpreted as pointing towards Jeremy was soaked up and fed upon in a kind of frenzy.
But Jeremy was tied to the Farm under the terms of his father's will. He could only escape from his life of drudgery upon his death. Nevill had left his estate to June, so with a double murder that's how Jeremy would inherit. He had already told Dorothy Foakes
he wasn't going to share anything with Sheila, and so it transpired. Had the twins lived they would have been entitled to Sheila's share.
This is how Jeremy Bamber's mind worked, it's how it worked 31 years ago and it's how it works to this day.