Whether settlement of a property can be made in favour of a lady said to me married as third wife

My brother residing at Vandalur,Chennai, an employee of the Tamil Nadu Govt was married in 1980 with a lady of Ariyalur .Without getting divorce from her, he married another lady in 1981, a co worker of his office and started living with the second lady by sending his first wife to her parents at Ariyalur 300 Kms away from Vandalur. While he was living with second lady, he got an ex party divorce order in 13-6-1990 from the Court at Chengalpattu, declaring that the marriage between my brother and his first wife which took place in 19881 be dissolved under Section 13(1)(i-b) of the Hindu Marriage Act. After 20 years of married life with the second lady, from 1981 to 2000, on 9-10- 2000, my brother and the second lady separated themselves without approaching any Court by executing a Marriage Cancellation deed and got it registered in the SRO, Ambattur. In that deed they have mentioned that they are leaving on their own accord due to their old age and no children were born and they are leaving without any consideration of money etc. The second lady is still alive. While being so, my brother without getting legal separation order from the Court from the second wife, at the age of 50 started living with another lady of aged 46 from 2003. Our family had already cut his relation ship from 1981 and we are not aware of all the above facts from 1981 (after sending his first wife) and we do not know whether any marriages was taken place earlier or not. After his death on 10-4-2017, We came to understand that he had nominated the third lady for receiving pensionery benefits and amount standing at credit in the Banks etc in the status as his wife. Moreover he had executed a settlement deed in favor the third lady declaring her as his wife, transferring land and building valued Rs.1.5 crores at Vandalur on 12-3-2014 and registered it as Document No.3434 of 2014, SRO, Guduvanchery. Kindly advice me (1) whether the third lady can be treated as legally wedded Wife, family in the absence of legal separation orders from the second wife. (2) If the third lady cannot be termed as a legally wedded wife, the settlement deed has to be treated as a conveyance/ sale deed and stamp duty of Rs.10,50,000/- at 7% of the market value of the property, together with Rs.10,500/- at 1% as registration fee should have been paid, but Rs.25,000/- alone has been paid as Stamp duty. (3) If the deed of settlement has been considered as stamped in sufficiently, kindly clarify whether the document can be treated as a valid Settlement deed or not and the course of action to be taken by me in this regard.