I know a lot of addicts will go to the hospital and fake pain to get a script for Vicodin. I'm not sure if that's what she is doing. I think at this point you could just focus on her behavior and not try to guess what's causing it. It is what it is and the kids need to be protected. I think this would be a good time for you and your sil to sit down and make a plan of how to handle the kid's safety.
It's easy for us to slip into denial in a situation like this. Denial can be an unconscious coping method. We get hit with some truth and then let it slide until we are hit with it again.
Here are the facts as I see them from your posts.
The children called saying that they are hungry.
They called looking for their mom after days of being left.
Your granddaughter called from school looking for somebody to pick her up from the school after her mom was an hour late.
Your daughter is taking your grandchildren into areas that are so dangerous it's not even safe for your sil to go there.
They are being left with strangers in that area which puts their life in danger and leaves them vulnerable to sexual abuse and all kinds of danger.
These facts lead me to think that the kids are in an emergency situation and need emergency intervention. The kids are dependent and rely on adults to protect them. If no one steps in to protect them they are defenseless and at the mercy of predators and abuse. It would seem to me that having them placed in that situation even one more time would not be acceptable.
This is when an emergency plan needs to be put into place now before something horrible happens to the kids. It doesn't sound like your daughter can safely care for her children right now. I will only use CPS as a last resort, but this situation is looking like it might fit into that category unless you and your sil can come up with an immediate plan to prevent this from happening again without CPS.
I'm writing this post as if I were in your situation and this is how I would process the information. I know how hard it is and know how stressful it is.
Hugs,
MG