It is up to the organization that you have to deal with as to what authority they require. The Land and Property Information Office which controls the ownership of real estate and therefore buying and selling of houses insist on a Registered Power of Attorney....

No - a Power of Attorney can only be given by a person who understands what they are signing. If she has lost the ability to understand what is going on then she can not sign a Power of Attorney. Your only solution is to make an application to the Guardianship...

Anyone over 18 years can be your attorney. It should only be given to a person who can be completely trusted as they can do anything you can do financially such as sell or mortgage your house or operate on your bank account (unless there are limitations against this...