How do I overturn a power of attorney?

I singed a power of attorney with and for my father October 2006. My father became quite ill with Allsymers and my mother is his carer - I presumed my Mother was dealing with his money but have recently found out that my Sister signed a new power of Attorney 2009 and is refusing to pay for anything she thinks is not necessary, she does not help with the physical care of my father, in fact I check on my parents daily where she only has contact when My Mother is when she phones them. I do not believe that my father was of sound mind when the power of Attorney was signed with my sister in 2009, Can I revoke the power of attorney and use the one that I had early as my sister is defiantly not looking after my fathers interests.