I'm confused .... 3 minutes is a VERY long time. I live on a very quiet street in Rockville and have never left my child on the sidewalk for 3 entire minutes. To put this in perspective, put your stroller back in the spot that she left your Child (and leave something extremely valuable in the stroller - like your wedding ring, of course this Does NOT compare to your child, but I want you to get even a sliver of the feeling that your nanny should have had when she left your baby on the street), walk in to your house, and wait 180 seconds before you even peep back outside to look at the stroller ....
If the police were able to investigate without either your nanny or sister knowing until they came back outside this means that they did not even do quick checks / peeks to see how the baby was doing.
Also, if your sister was there, what the heck was your nanny doing for 3 minutes with your older child? Why not give the older child to your sister and run back out to the baby? She sounds way to comfortable to me and her decision making on this would cause me to never fully trust her again.
Lastly, who the heck leaves the baby by the sidewalk??? I'm not sure how your house is situated, but if you were going to run in to the house, at the very least - leave the baby right by the door.
Net net, even if you do keep her as your nanny, I am not sure how you could ever trust her again.