Check the wording, very, very carefully. I was surprised when rereading my father's that I actually have authority to appoint someone in the event I can't act or need a co-proxy.
Whether or not your sister can be added depends on whether there's an authorization in the POA provisions.
Whether or not your sister can be added depends on whether there's an authorization in the POA provisions.
Your mother can, though, as Jeanne explains, provided she has the mental capacity to do so.