They are valid if you want them to be. However, you *cannot* be forced to be an attorney-in-fact (that's the term for the person granted power by a POA) against your will, so you don't have to function in this capacity if you don't want to. You should both speak to your sister, brother, and mother and also send them something in writing, sent some way that you can prove delivery, letting them know that you did not agree to be the attorney-in-fact and refuse to serve in that capacity, so they should select someone else.