The will is valid. Your father might have purchased in the name of your brother but it does not mean that it belongs to your brother, as it was purchased by your father and his Will will be final, provided the Will is validly executed.
This is known as a Benami Transfer. Where one person pays the consideration (real owner) but the property is brought in some other person's name (benami owner), this is illegal practice, and the benami owner becomes the real owner for all purposes, which in your case is the brother. However to circumvent this sub section (3) of S.4 of Benami Act would come to your rescue. See a lawyer a person who would help you understand better.
Conveyance of property can not be revoked even it is benami ( person is not fictitious in this case). I think, property- holder-brother should transfer by way of conveyance respective shares to other legal representatives by paying actual stamp duty to fulfil the wish of the father. If he does not want to transfer, then other brothers are unlikely to get legal relief.