Thanks everyone, great replies and have given me some food for thought.
I think what you said @atomicmama about the situation with farmers is spot on and I'm sure that's what my sister is feeling - that she's put a lot of work into something and I'd just waltz and and be a part of it after having nothing to do with the business at all. But then on the other hand I do understand that Dad was trying to build a legacy for both of us and that he always had me in mind as a part of the reason why he was building a business that may have long-term residual income, meaning that the work is done now and may potentially keep on paying for years later without any more work being put in, ifykwim? In that situation my sister would see that as me getting "something for nothing" rather than a gift from Dad that she is also receiving equally. I do understand her perspective and likely I'd feel a bit the same if the roles were reversed.
It is quite awkward to sit down and talk about with the three of them as my sister doesn't feel that it is any of my business, so it's between her and Dad. I did say to Dad that I'm not attached at all, and that he must do whatever it is that he wants to do and certainly, to take into account my sister's feelings on the subject, but ultimately it is his decision and I'm fine with whatever that is. If he does leave me part of the business, then I'll need to have an idea of what to do - as others have mentioned above, my sister might think that I want her to buy me out at market value or something and be worried about that. I wouldn't do that and I don't have any interest in being involved in the business myself, so I will have to have a think about what would happen if Dad did leave me 25%.
Ideally yes, she would buy Dad out now and then his cash is his to do what he likes with. I might even suggest that to him now, (only if he brings it up again) as an option, especially if his health is deteriorating and he can't work full time, then at least he can have peace of mind that he's out of the business and can make his own decisions without upsetting my sister.
@VicPark that's exactly what I did He doesn't need stress right now, just support, so I'll give him that and even though I do think my sister is out of line, in the great scheme of things I never expected anything from my parents anyway, so it's neither here nor there, but I don't like seeing Dad being bullied into a decision that he might not want to make in quite that way.