Legal heir certificate is susually issued by O/o Tehsildar, post inquiry by designated officials.
Some states/establishments might have provisions for Family Tree certificate and information on state govt website that issues such certificate should be available. Check and reconfirm from authority that issues the certificate.
Succession certificate is issued by Court.
Check and reconfirm from authority ehat exactly IT wants?
If legal heir affidavit attested by Noatary or Ex. Magistrate can serve the purpose and establishment accepts IT then obtain response of establishment in writing and you may go for IT and submit under proper acknowledgment.