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There is a certain presumption within the legal system that parents have better rights to their children than anyone else, including grandparents. You may be able to rebut that presumption if the NCP for the previous seven years has had very minimal contact with his child. However, your post suggests that Mom and Dad never officially divorced and that there were no custody or support orders in place before Mom died. Therefore, proving all of this is going to be somewhat hard.
Consult local counsel.