your great grandmother
Your mother's mother would be your grandmother.
Your mother's mother is your grandmother.
your mothers mother is called your grandmother. and your mothers father is called your grandfather.
I call my mother mom
You can call her by her first name or if your comfortable some son-in-laws call them mom.
If mother is singular: mother's If mother is plural: mothers'
Mothers is more than one mother. Mother's is something that belongs to a mother. "The mothers group is meeting Tuesday." "My mother's car is in the shop."
Your half-sister's mother may be your mother, if you have different fathers. If you have different mothers and your father is married to your half-sister's mother, then you probably should call her "mother." If your half-sister's mother is not connected to your father (i.e. her father married your mother) then there is no relationship name for you to call her mother, other than "my half-sister's mother."
You would use "mothers" for two, as it is a plural noun.
Mother = äiti. Mum = äiskä, äippä, äityli, äityliini, äite, äitee
The plural is mothers-in-law. (not mother-in-laws)
The possessive form for the plural noun mothers is mothers'.Example: The mothers of several students have formed a mothers' committee.