It's my understanding that whilst anyone can attach conditions to their wills, it's entirely up to the recipient whether or not they're carried out.
I realise that, but it's putting pressure on the recipient, and if they don't like the conditions they have to choose between the dosh and doing something they don't want to do.
I don't know if that was Nevill's intention. It might be that he thought Jeremy would need more money than Sheila if he was going to keep the farm running.