Question regarding the eligibility of property rights

A 1 storied house built in the year 1990 and was named after the mother in the house as the rightful owner. However, in the year 2003 the original deed of the house mysteriously disappeared. No FIR was filed. The owner passed away in 2005 leaving her son and his family living in the house. With two other daughters married off before the passing away of the mother. Forward to 2014, the son now living in financially dire condition decides to sell the house. However, the youngest daughter of the deceased mother produces a will, wherein the daughter was named as the rightful owner under the circumstances of her being unmarried in the year 2003. All of it being done without the oldest son's knowledge. However the daughter has never been able to produce the will anywhere else apart from the selling of the house where she demands 90% of the money from the selling to be given to her. So my question is, can the daughter actually have rights on the property since she was married off just the next year of the will being made? And, she has never claimed anything related to the house elsewhere apart from in 2014 when the house was about to be sold? Are there any chances that the deed or the will that she holds has been forged? Considering the mother in the house was mentally unfit during the time of the making of the will.