Will my situation be adoption?

My sister-in-law had drug issues for many many MANY years. She lost custody of all her children at the time of birth. They were all born addicted to drugs. Many have been adopted outside the family. She had 12 all together. My mother-in-law legally adopted 2 of the children. She has recently been put into hospice. We will legally take over caring for the children when she passes. Will we be considered adopted parents or guardianship? Not sure how all of this works. Do we have to go through a special process?