Accessing bank acccounts for deceased person

[Concerned niece]

My spinster aunt passed away a year ago. My estranged sister was her POA for the last 4 years of m aunt's life, and has not disclosed to any family members (including my father who was next of kin) what was in my Aunt's will, or how her accounts were handled prior to her death. I have applied to the Public Trustee in WA for a copy of the will, but am trying to see what happened during the past year of my aunt's life (when she had severe dementia). I recently learned from my sister's daughter, that her mum had given her a sum of money out of my aunt's account .....however, my sister is refusing to confirm or deny this. I have contacted my aunt's bank, and they cannot provide any of the old statements as I was not POA. I am POA for my father (who is my aunt's closest living relative), and am trying to investigate this on his behalf. I do not know how to move forward if the bank will not release the information. Any help, or advice would be greatly appreciated.