What should I do if my son's maternal family and his step dad is keeping her away from me?

Mother deceased, never amended birth certificate to reflect my name despite order to do so. Her husband or mother-in-law will not tell me where my child is. I have been trying to get her for two years. She moved to Idaho and was ordered to come back to California with my child and did not. Now she is deceased,I am getting mixed information from many sources. Idaho says I cannot get her even if she is there without a court order of custody, California says I have all documents needed and police will not search for her because I don't have what I need.