Mother's: something that belongs to a mother. Example: That's Mother's perfume.Mothers': More than one mother.
Singular possessive: mother's milk Plural possessive: mothers' milk
When your mother's mother and your father's mother are sisters, then your mother and father are first cousins to each other.
You may be. Technically, if your mother is Jewish you're Jewish. So, if your mother's mother was Jewish your mother was Jewish and you're Jewish. However, if your mother wasn't raised Jewish and/or...
Your mother's mother would be your grandmother.