Yes well said from everyone. Perhaps reevaluate where Mary Donaldson came from? Who told you this or where did you see it? Was that perhaps where the mistake was made? Obviously Anna McDowell was someone that Mary Ann spoke of as being her mother, or similar, or I don't believe her husband would have listed this name. Perhaps yes, she was a step sister instead of biological sister to your grandfather, or even cousin. Death certificates can be some of the most misleading documents in our field and always seem to necessitate further investigation than we expect. I would be interested to know the outcome of what you find.