How do we add my sister to the power of attorney we already have for our mother?

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.

You don't.

Your mother can, though, as Jeanne explains, provided she has the mental capacity to do so.

You can make out a new document for your mother to sign. (I'm assuming she wants your sister to have POA?)