The proof of will provided under Section 63(c) of the Indian Succession Act which says "The Will shall be attested by two or more witnesses, each of whom has seen the testator sign or affix his mark to the Will or has seen some other person sign the Will, in the presence and by the direction of the testator, or has received from the testator a personal acknowledgement of his signature or mark, or the signature of such other person; and each of the witnesses shall sign the Will in the presence of the testator, but it shall not be necessary that more than one witness be present at the same time, and no particular form of attestation shall be necessary." and Section 68 of the Indian Evidence Act which says "Proof of execution of document required by law to be attested.—If a document is required by law to be attested, it shall not be used as evidence until one attesting witness at least has been called for the purpose of proving its execution, if there be an attesting witness alive, and subject to the process of the Court and capable of giving evidence: 1[Provided that it shall not be necessary to call an attesting witness in proof of the execution of any document, not being a Will, which has been registered in accordance with the provisions of the Indian Registration Act, 1908 (16 of 1908), unless its execution by the person by whom it purports to have been executed is specifically denied.]"
In so far as the essentials of a valid will is concerned, Section 63(c) says that the will SHALL have two or more attesting witnesses.Therefore attestation is required to have a valid will.
Now the next question is, HOW to prove a will? - the mode of proof is given in Section 68 of Evidence Act - which says that if a document is required by law to be attested, then at least one attesting witness has to be examined to prove the document.
However, there is a proviso(or an exception to the rule) - which says that if the document is registered and the execution of the document is not disputed then the mandatory requirement of examining the attesting witness is not required.
In your case, your brothers have disputed the execution of the will and hence, though it is a registered document, examination of the witnesses is required under law.