First, this was a very thoughtful question. I feel for your son.
We are not in this same position so I will let those who have had caregivers vanish deal with that aspect of it, but I wanted to note: Is it possible that her "personal/family issues" were creating a real crisis? I am assuming here, maybe wrongly, that you have at least an idea and possibly detailed knowledge of what those issues are. Even if you don't, the fact that she did not answer calls or texts and it's "uncharacteristic" of her, all sounds as if anything could be going on -- an abusive/controlling situation at home, grave illness (hers or someone else's), legal issues that have made her try to drop off the radar, anything. You mention "taking care of her daughters" and it's not clear if that is just a way to help interpret this for your son or if there is actually something wrong with some or one of her children that you know of.
I would be concerned about her, frankly, as well as about your son, especially as she vanished so drastically and dramatically. Now, if you know the circumstances, know there are issues but not something serious, you weren't really surprised, etc., that's one thing, but if it is the case that you don't know what happened at all, or know maybe that there was illness in the family -- maybe see if she's OK?
Alternatively, maybe she took another job and this was her way of quitting and she didn't want to or didn't know how to do it properly....