im listed as the beneficiary on my fathers bank account. is that enough or do i need a poa ?

i have been taking care of my father for close to 10 years now. I have 3 older brothers who never helped me with anything regarding my father in his old age. one of my brothers is on drugs really bad and has been abusing drugs for over a decade, my other two brothers were never around , but recently has been coming around. my father was diagnosed recently with moderate dementia and can no longer take care of himself at all. I have taken my father to every doctors appointment and taken care of him on my own. I am listed as the beneficiary on his bank account and I also have documentation from the neurologist who diagnosed my father , that when asked who does he want to be a POA over him and my father named me . im scared my brothers will just bully me as I am the youngest of my fathers children, is being listed as his beneficiary on his accounts is this enough? and what exactly is a beneficiary on an account?