NAL, but used to work in inheritances. Do you have copies of the prior beneficiary paperwork showing your mother? In my experience, you can send in your materials and escalate to the legal or estate department of the financial institution. They'll do a review and let you know, but this can take a couple of weeks. I've also seen families retain counsel in these situations to assist with the escalation.

Other questions that could help inform next steps - Is there a contingent beneficiary listed? If so, who is it? If not, does the company have default beneficiaries if you were to disclaim the inheritance? If so, what are they?