Parental rights: my ex wife wants to relinquish her parental rights to my mother who has guardianship of the kids but I want to retain my rights can this be done

Its unlikely the courts will allow her to relinquish her rights unless your mother is willing to adopt. However, to clear the way for adoption its likely you both would need to relinquish rights.

So I would just let things be. Your ex does not have to exercise her rights which would have almost the same effect as relinquishing them.