Hmm yeah...could she not have texted your sister instead of going inside and leaving baby outside? Was the stroller on or near the porch or entrance of the house, or was it left on the sidewalk or far from the house? Either way it isn't good, but, I think if you are considering not firing her over this incident, I would only do that if her reasoning was solid (she couldn't have texted your sister for help) and if the baby had at least been removed from the sidewalk and placed in a safer location adjacent to the house.
I care for infant twins and when we prepare to leave the house in the stroller I set up the stroller just outside the front door of the house, about 20 feet from the sidewalk. I put one baby in a safe place (swing or bouncer) inside the house, and I bring the other baby out and strap him in. Then that baby waits in the stroller for about 20 seconds while I go grab his twin. It's not possible to carry both twins and the diaper bag safely while exiting through the metal screen door which is on tight springs and slams back quickly. I've actually spoken to their parents about this, and they agreed that when one of them was with the twins alone they had to do it the same way also.
Of course, you're talking about one baby and no twin, but if it was for less than 45 seconds and if the baby was at least in a safer spot than just on the sidewalk or adjacent to the sidewalk, then if you're otherwise happy with the nanny, perhaps you can give her a stern talk about how this is NOT OKAY and maybe keep her on on a probationary basis or something?