I think it is time to get children’s protective services (CPS) involved. If you don’t deal with this issue proactively you are likely to become depleted and overwhelmed, and not be able to care for them in the long run.
Let CPS know what you are struggling with and find your daughter-in-law’s caseworker. He or she may be able to help offload the burden of responsibility from you. He or she may also be able to help you manage legally and in the coordination of services. Take a strong proactive approach to getting as much responsibility back on to the children’s parents as is possible with ancillary support services from CPS.
What I am suggesting is to start finding ways to deal with the fact that you have to have others take more responsibility so you can take better care of your needs. If you become depleted and can’t care for the children what would happen? That is the scenario you must use to guide your need for greater self-care. Ultimately you may be in a better, stronger position if you can be a support person rather than primary.