Father has custody of Daughter.
Father marries Stepmother.
Stepmother is only mother Daughter has known since she was three or younger.
Daughter is now 32.
Stepmother recently became ill.
Daughter decides to formalize what she feels.
Stepmother adopts Daughter.
Stepmother becomes Mother.
More than half of the states in the US permit an adult to be adopted.
Stepparent adoptions, in particular, are fairly common.
When the prospective stepchild adoptee is a minor, however, the consent of the other biological parent is typically required.
So, many such adoptions are deferred until the child becomes a legal adult, at which time consent of the other biological parent is no longer required.