@EllaC s117 of the Succession Act 1965 refers to parents making 'adequate provision' for their children in their will.
This is the basis for contesting a will and the estate pays the costs.
Therefore if she knows approximately what the assets are (house + bank accounts etc) she will know the value of the estate.
It wont be too late at probate. Though conversations with brother might help as he may be already aware of this from his legal adviser and maybe this will bring both parties to their senses - life is too short.