Although I agree that you need legal advice, it is of course, your decision as to how you want the financial /property side of things sorted out. The solicitor can draw up the agreement and the wills, but the basic decision is yours.
Beau's advice and experience are interesting.
I would try to leave any 'care' considerations out of it, and make no demands or have any expectations.
I would also deal with the 'possible split' issue separately to the inheritance one.
Begin by considering what value, if any, is added to the house by the annexe. This is actually difficult to do until it is built, but a reputable estate agent will give you an approximate estimate.
You don't say what kind of annexe. It may well be possible to regard it as a separate dwelling, with say, shared use of the garden, shared drive etc. - and this is easier legally, as each property can be valued separately. You can then deal (possibly with a lump sum) with the decrease in value of the main property. You will of course, be liable to separate council tax etc.
However you do your sums, please remember to divide the inheritance as a share, or 'sum equal in value to current valuation' or similar. My grandfather made a will leaving me a sum of money that would have bought half a decent house. By the time he died, it was approximately a week's wages. I'm certainly not complaining, I just know it wasn't what he intended fifty years previously!