Apologize to your boss one more time (it can't hurt). But after that, no more. You did your best to get there, and your sister was stuck in traffic and you could not leave a child unattended.
After that, I'm not sure, but I seriously doubt that one unexcused absence will allow the yanking of your certification. Surely there is recourse in such a situation - and there is no way that anyone is going to ding you (and make it stick) for not leaving a child.