Parents adopting grandchild?

My father remarried over 10 yrs ago. About 3 yrs my stepmother adopted her grandson. His parents were on drugs and lost custody. My dad never signed any of the adoption papers but is raising the child. My stepmother is not in the best of health and I am honestly worried she won't live to see her grandchild's 18th birthday. My question is....if she died would my dad be given custody?