Your father shall be the title holder since he has not executed the transfer by a registered deed in favor of your brother.
The society's transfer of share certificate to your brother's name is not a title document in favor of your brother. It is just a transfer made on request, however since there is no transfer made by a registered document, your father remains the absolute owner with clear and marketable title, he can apply for cancellation of the transfer on the basis that the transfer was not effected by a registered deed till this date.
The property still lies on your father's name only hence he may have to get the transfer certificate now lying on your deceased brother's name cancelled.
Your sister in law or even you do not have any rights over your father's property, at least not during his lifetime.
Your father may be 86 years old, but when there is a litigation, the age does not matters, the law is common for all.
Hence he may better enter into a compromise if at all he wants to avoid unnecessary legal hassles or litigation problems.