Is accessing someone's online banking using their credentials to transfer yourself funds actually illegal? - England

Slightly complicated but I'm trying to work out legal implications and looking for advice. My mum died a few weeks ago, and in the month prior to that she was barely able to communicate. My sister had access to my mums phone with her online banking during that time and transferred herself money from my mums account while she was virtually unconscious She claims it was all agreed and they talked it over (my mum could not talk!) as it was money she was owed which i do not believe for one second. Regardless if it was, there must be laws about this If someone accesses an account using someone else's credentials to pay themselves money while the account holder is in hospital and unable to consent isn't this illegal? Does anyone know?