DH's sister has gone ahead (without consulting him) and set up Power of Attorney for their mother (who's been diagnosed with dementia). DH gets on well with his mother, but she's not in a state to really know what's happening. His relationship has been tricky with his sister over the years, but they do speak and we thought got on OK. Is she allowed to do this? Can it be challenged? She has also set up online banking for their mum and won't share the access info.