Since the property is registered in the name of the Mother, her will to transfer the property is paramount. Being the owner of the property she can do anything with the property. She can also execute a will by which she can transfer the entire property to either of her two children or to a complete stranger. The Brother cannot sell the property till he does not have the property in his name.
The Brother will get the property only in the possible scenario where the Mother dies without a will when the property will be shared equally between the Brother and the Sister. Even in that situation, the Brother cannot sell the entire property as the Brother and the Sister will be joint owners and can execute sale deed of the property only up to the extent of their share and not beyond.
The ideal solution will be that either the Mother sells the property to a third person or to the Sister
Execute a Gift Deed giving the property to whoever she wishes to
Execute a registered will determining the ownership of the property after her death
Researcher - Saumya