Claiming policy

I a having a policy with Safrican from 2020 now one of my extended family member, my sisters child who has a different surname as mine has passed away. I am now requested to do letter of authority and the deceased asserts are more than 250 000 which needs a letter of executorship. The deceased has left eveything to her mother who is the one who will be doing letter of executorship and not me. I am failing to understand why i need a letter of authority because thats not my child. Safrican just needs an excuse of not paying thos claim. The claims assesor was Nomfundo Zondo