Nedbank - issuing of power of attorney without family knowledge

My dad was admitted to Sandton Mediclinic in September last year. He was extremely ill and unfortunately passed away on the 8th of October. A couple of weeks before his death, we were horrified to find out that Nedbank issued his sister's daughter with the form for power of attorney. She then went together with her mother and brother and got my dad to sign this document. This was done without the knowledge of any of his own children. And just to give you an idea of how sick my dad was - he had no idea of where he was.....he had no memory at all for example, he had no idea what his phone number was or even his age. he couldnt even tell the difference between night or day. He was also suffering from some sort of dimentia. When this document was signed in the hospital, there was no doctor, no representative from the bank or any other person present. It was only the three of them there and they are the only three people that signed that document. The doctor that was presiding over my Dad was just as horrified and even wrote a letter stating that my Dad was not of clear mind to make any decisions regarding his estate, will or finances. We have an official letter stating all of this. Subsequent to this, we have now had insight into his account over that period and there are transactions that seem to be **********. We are seeking legal council at the moment as we want to know how can something as sensitive and important as a power of attorney been given to someone that isnt even directly related? Also, how can this be legal without any professional; person being present to oversee this process. As far as i am concerned, this was obtained in a ********** manner and we have proof that there were ********** transactions on his account. Nedbank, you will be hearing from our lawyers.