1. you first need to check whether or not this property is subject to the provisions of the Public Premises Act since originally the building was meant for occupation by the staff of SBI
2. ideally on the demise of the original shareholder, the society ought to have requisitioned an heirship certificate or a letters of administration to be produced by the widow of the deceased shareholder, before transferring the shares to her name
3. however such an application as above cannot be made now since the widow is no more. the aforesaid would have ensured that apart from the widow there are no other claimants claiming any right over the flat
4. the widow died testate by leaving behind a registered Will
5. the Will ought to be probated. even though there has been no claim since after the widow's demise, a probated Will would ensure that the widow really intended her flat to go to her sister and BIL, as a probate grant is a certificate by the Court that the Will is genuine and is made as per law
6. please note that just because the Will is a registered one, that alone would not support its authenticity and genuineness. even a registered Will has to pass the muster of the law just like a non-registered Will
7. it does not matter that the shares are already transferred to the beneficiaries of the Will. In absence of a probate grant, such transfer still remains questionable and is amenable to be set aside in future
8. so it would be in your interest that you tell the seller to first have the Will duly probated and only thereafter it would be safe to proceed with the transaction
9. the probate grant is also necessary from the future's perspective since any buyer of yours would obviously ask for probate
10. as regards the earlier transfer in favour of the widow, you will have to issue a public notice and invite objections and claims
11. to repeat, do first also check how this property became a private property owned by a co-operative housing society when originally it was owned [i assume] by the government, as the old premises were allotted to the SBI staff