Interesting land dispute and unregistered sale deed of 1998

A piece of land of are A is owned by 3 brothers and a sister. One of the brothers is deaf and dumb and illiterate and not married as well. The remaining 2 brothers sold a portion X of the total area A to their sister via an unregistered sale deed in 1998 and verbally promised her that they would sell the remaining (A - X) area of land to her in future and she can get the whole area A registered in her name at that time. Now in 2018, without informing her, the two brothers sold the (A-X) area of land to a third party and also registered the sale deed without her signature. They are also trying to sell a portion of the area X that they sold to her in 1998 taking the advantage of the unregistered sale deed. The sister is already in possession of the X area of land since 1998 (from the time she purchased it) and since this is a commercial plot she has done some enhancement and is also giving this plot of area X on lease to some other people since the date of purchase which is 1998. Now there are three Questions 1) Even though the sale deed committed in 1998 is unregistered (which is before the amendment that came in 2001), can she hold the possession of the land of area X through Section 53A of TPA and through Proviso of section 49C which talks about Part Performance as she took the possession of the land as per the contract and also have done enhancement on the same ? 2) Is the registered sale deed that her 2 brothers executed for the (A-X) area of land without informing her and without taking her signature valid ? 3) Is the thumb impression of the brother who is deaf and dumb and illiterate and does not understand anything except some specific signs like sleeping and eating mandatory in any of the sale deeds as his thumb impression can be easily taken by anyone ?