Clarification on Will left by father

My father died on 6.07.2004. My father left a registered Will on 2.06.1995 for his self-earned house property at Tiruvottiyur, Chennai-600019 to his wife and she shall only have a life interest with no power to encumber or mortgage the same in any manner whatsoever. On her death or on her remarriage the said house and all the movable property not disposed off, therein shall revert absolutely to his ony son.(Myself) Though he had bequeathed no share in his properties to his two daughters, as a token of love and affection for them he directed his wife and son that they would pay to each one of my daughters a sum of Rs.10,000/- i.e first daughter will get Rs.20,000/-(Mother’s contribution Rs.10,000/- + Son’s contribution Rs. 10,000/-) in the same manner 2nd daughter will also get a Sum of Rs.20,000/-.The said sum to be paid within 2years of his demise and in case of default, they will pay the same with 12% per annum with quarterly interest. Now still my mother is living and my two younger sisters are demanding equal share by asking my mother to forego life interest in that property. Whether it is correct in the legal point of view? Kindly clarify?