Beneficiary died one day after the testator and the will is unregistered.

My uncle was a bachelor and had raised my mother since she was 14 after their parents died. as such he wrote a will two years ago declaring my mother as the sole beneficiary. he didn't register the will, though. my uncle was third in four siblings with my mother being the last. the other two have died years ago and they have two children each. he died last month after being sick for a while. my mother who was hospitalised as well expired the very next day. their combined medical bills have made a significant dent in my finances and my cousins had not pitched in a dime. while he was alive the plan was to sell his flat to cover the expenses as he stayed with us. after his death i came to know that he had quite a bit of money saved up in his fixed deposits enough to push back the sale of his apartment. he had placed my mother as the nominee in all of them. however, i am being told the will cannot be accepted as it unregistered. and i need to get legal heir or succession certificate to access the accounts as both the depositor and nominee had died. what is due the process i should follow?