1. it appears your father in law does not have a registered title document in his name from the seller [the old man] from whom he purchased the flat
2. it appears that the seller only gave a POA to your father in law and then somehow your father in law's name came to be entered in the share certificate
3. mere entry of name in share certificate is not enough and does not confer any ownership title on your FIL
4. since your FIL has passed away, things have become more complicated
5. on his death, his legal heir has to first obtain letter of administration [LA] from Bombay High Court which is an expensive and lengthy procedure of 6-8 months
6. once the LA is granted, a registered sale deed has to be made by the old man in favour of the administrator and thereafter by the administrator in favour of the mother in law
7. alternatively the old man can directly execute the registered transfer deed in favour of your mother in law with the administrator as the confirming party
8. this registered title document will give title to your mother in law
9. however you also need to check whether NOC of SRA is required for the above transfer since the original transfer by old man in favour of your father in law appears to have been made by him without NOC of SRA
10. only on getting the above registered title document, can your mother in law get ownership rights in the flat and be able to sell the flat
11. for the transfer deed, it is advisable that the mother in law takes a POA from the old man [however this POA will be valid only till the time the old man is alive]