I'm just wondering, if the land is valued at 250k, then I guess it has at least Outline Planning Permission?
Even I know that what Barmaid has said is so true. It's going to be a struggle to come up with a fair division......... one that your brother will agree to.
As was said, an acre is a big plot. Half an acre is also more than enough. Why not go back to the Planners with a proposal to have PP for two detached houses. From a Planning point of view, it shouldn't be a problem. Agree with your Mother that you should each be gifted a plot each. He can do what he like with his, and you can sell or keep your plot.
I think that would prove to be much less of a problem legally.