Airtel — Deceased person's SIM card ownership change

My brother has passed away last month and my father was holding his SIM card until last week. but we came to know that, one of the legal heir has approached the Airtel store and transferred the ownership on his/her name. this my father came to know after the SIM got deactivated.
how can the ownership of the deceased person's SIM can be changed without the NOC certificate of the other legal heir.