This makes Benjamin, Mary's husband, her first cousin. You can see why Franklin Rice dismissed that as not possible in 1908 when he did his research.
Mary's gravestone is in excellent condition and is not as "primitive" in appearance as her husband. She died in 1736 and her husband died in 1748 so it is possible that her gravestone was carved and placed here after her husband's gravestone.
Her birth record indicated of day of the week and only give a "first week" of August as part of the record. This also may have added to the confusion over which Mary this was.