My Mother is the best mother in the world - admittedly she is my only mother, but she is not bad .
When your mother's mother and your father's mother are sisters, then your mother and father are first cousins to each other.
Singular possessive: mother's milk Plural possessive: mothers' milk
Mother's: something that belongs to a mother. Example: That's Mother's perfume.Mothers': More than one mother.
Your mother's mother would be your grandmother.