Deed of confirmation by the vendor is subject to payment of stamp duty.
Section 213(1) read with section 57 of the Indian Succession Act, 1925 inter alia states that the right of a person who inherits a property bequeathed under a will cannot be established unless a court of competent jurisdiction in India has granted a probate of the will or letters of administration with the will annexed thereto have been obtained.
It is pertinent to note that the declarations executed by other legal heirs of your mother on stamp paper—stating that they have no right, title or interest in the flat in question—is not sufficient to establish your right over the flat and the same can be subject to challenge at a later date. Thus, it would be advisable to obtain a probate of your mother’s will.
You can apply for a probate of your mother’s will in a court of competent jurisdiction. The maximum court fee payable in Maharashtra is Rs.75,000 and it takes 8-10 months to obtain a probate. It is to be noted that only the executor appointed under the will can apply for a probate. If there is no executor appointed or the executor has passed away or does not wish to perform his/her duties, you may apply for letters of administration with the will annexed.
The unregistered sale deed is admissible as evidence.