I'm sorry you're dealing with this. You sound like you are patient and insightful with your son!
I don't know if I'd 'confront', but maybe rather e-mail her a very non-emotional request that if she is unable to take care of the children, she will leave them with you and not in the older daughter's care, and that you are told about hospitalizations and similar situations, just as you tell her when you are out of the country and such. And end it 'thank you'.
That way you have it documented and you avoid the potential for a verbal dispute.
M 16 yrs, WH62, P54 3 adult blended kids EA 11/13, BD1 6/14 PA fall 14, BD2 2/15 Piecing 2015, BD3 12/15 Separated 4/16 WH moved OW in 5/16 Divorced 6/15/17