Sister abusing power of attorney

Hi there,
My sister who is power of attorney for my mom who had an annyersum 20 odd years ago will not answer questions I have about my moms finances. I am worried about how my moms finances have been managed with large sum sof money gifted to my sister over the years and Im now trying to advise my Mom if her home she rents out is actually making her money or she is losing money and I cant provide this as my sister refuses to give me specific details on outgoing saying I am not entitles to thins infromation and has blocked me completely. What rights do have to insist on her provid this information please?