Hi Good day!
In Mumbai, Legal Heirship Certificate can be obtained from High Court of Bombay only not from Tahsildar office.
For the grant of Legal Heirship Certificate, you need to file petition in High Court of Bombay and adequate court fees (₹75,000/- in most cases) needs to be paid.
It's a time consuming process and generally it takes around 6 to 12 months.
In near future if your mother decides to sell the property Legal Heirship Certificate won't be sufficient.
For selling of the property you need to get Letter of Administration from High Court.
My advice for you is,
1) Please check who is the Nominee of your flat in society records.
2) A valid Nominee is enough to transfer the flat in society records.
To transfer the name ownership of flat:-
1) Register Release Deed of yours and your sister and get the property transfer to your mother name. Please note that there is stamp duty payable on Release deed which is minimal.
Get Letter of Administration from High Court.