Your grandmother's sister is your great aunt.
Your grandmother's sister's children are your first cousins, once removed.
Your grandmother's sister is your great-aunt.
Your mothers grandmothers children are each others siblings.
Yes, your great grandmother's sister is your great-great aunt. Your great aunt is your grandmother's sister.
My ancestors
1st cousin 2x removed
Your sister would be your grandson's great aunt.
She would be your niece.
That depends, if you are referring to grandmothers as in a group or multiple grandmothers no, if you implying it belongs to a singular grandomother it would be "grandmother's" and finally if its something possessed by multiple grandmothers it would be "grandmothers'"
Your sister's daughter would be your niece. You would be her aunt or uncle, depending on your gender.
Your wife's sister would be your sister-in-law.
You would be the Aunt or Uncle.