Your mother's brother is your uncle.
Your mother's brother-in-law is your uncle.
you call him your uncle
Your mother's brother is your uncle; you are his niece (if you are a girl) or his nephew (if you are a boy).
Your mother's brother's son is your cousin.
they are your cousin
Wouldn't that be an uncle.....
Uncle
Your mother's brother-in-law is your uncle.
Your mother's mother is your grandmother.
Your Grand-mother's brother - would be your great-uncle
You don't call your brother's sister your aunt. Your brother's sister is your sister, or you. Your aunt is the sister of your mother or father.