Your DPs family need to be weaned from total dependence on him. It is time he had a rest from paying for everything and doing everything for everyone.
Start with housework. The ones living at home are old enough to clear up after themselves, cook a proper meal regularly (once a week each at least?) and do their own laundry. Pushing a vacuum round is not beyond their abilities, either, or cleaning the bathroom.
Getting the son onto benefits would be a step to sorting him out a little. He could then pay something toward his keep, and have pocket money to spend without relying on his father. He can be expected to take charge of his life and do more than sit playing computer games. He might even end up working, who knows!
The daughter may be saving for her own house, but if she pays nothing toward household expenses, she is doing at on her father's money. She should contribute something - the cost of her food ath every least, and ideally a proportion of the other bills.
The other daughter is a bigger worry, because she has an emotional lever to pull in the form of grandchildren. I agree with other posters that Social Services could help here, and the rest of the family working together to curb her selfishness may be able to get her to use a more responsible attitude to her children.
Whether they remind her of her ex is irrelevant - they are hers, and they rely on her. She is no longer a child herself, she is a mother, not a silly teenager who pays attention only to her latest man.
You have your hands full if you try to sort out this disfunctional family yourself, greytemples Get all the help you can.