I think you need Legal Heir certificate. This document lists out all the legal heirs that are entitled to a share of the assets. Once all the heirs are listed on that ,then only the No objection certificate comes into play here. Also the death certificate is the first document to be submitted. Lastly, the Power of Attorney authorizing your mother in law to act on behalf of all heirs to divide the assets or to act as she deems fit. These are the documents required.
Believe me the process is even worse if you consider share certificates in the name of a sole person now deceased. To add to the problem,the procedure is not standardized. Hence advise everyone to nominate some beneficiary now itself.