Your question has multiple moving parts. I will start with the question about your son-in-law because in most situations, it is the most simple solution to the concerns you have expressed. Your son-in-law could ask the court to allow him to adopt your grandchildren from your daughter's prior relationship. This would make him their father, including causing their birth certificates to be amended to reflect that he is their father. Then, if something happened to your daughter, the children could remain with your son-in-law along with their siblings without any real question. Also, unless the ex's parents ask the court for visitation in the adoption and it is granted, they will be out of luck. In the event that your son-in-law does not want to adopt your grandchildren, then, in the event that your daughter does pass away, either you or your son-in-law could petition the court for custody; however, the ex's parents or other family members may do the same.