I'm sorry to hear about what your children are going through. Would it be possible to attempt to get custody again based on the lack of concern/care for D's medical issues? Or the fact that S has to care for an elderly man who I am guessing has dementia or alzheimers?
This is neglect, plain and simple in my book. Although I am not a lawyer. Be prepared for the blow up between S and Dick. My S who is younger than yours finally exploded on H a few months ago and it was not pretty. Maybe if that happens, Dick will volunteer to send the kids to you. I don't know. I know of no organizations that help kids other than child welfare and I don't know if that is the route you want to take.
"Acceptance doesn't mean resignation. It means understanding that something is what it is and there's got to be a way through it."--Michael J. Fox