Your aunts' mother-in-law.
Your mother's half-brother is your uncle, or half-uncle if you want to be exceedingly precise.
Your mother's uncle, whether maternal or paternal, is your great uncle. You are his great nephew if you are male, and his great niece if you are female.
Your mother's brother is your uncle. Any child of your mother's brother is your first cousin.
Relationship sequence:Your uncle is your mother's brotherHis children are your first cousinsHis grandchildren are your first cousins once removed
Your sister's mother is also your mother.Your mother's brother is your uncle; and his brother is also your mother's brother, so he is also your uncle.Your uncle's son is your first cousin.
If your uncle is the brother of your mother or father, his mother is your grandmother and her father is your great grandfather. Your great grandfather's wife is your great grandmother. All presuming that there were no widowers or divorces and second marriages among these ancestors.If your uncle is the husband of the sister of one of your parents, then his mother's father's wife (his maternal grandmother) is not related to you at all.
Aunt or uncle and they nephew and niece, there's not other relationship term that can fit that relationship unless in a mother tongue.
Mother, father, sister, brother, aunt, uncle, friend.
Your relation to your mums uncle is GREAT uncle. You're welcome! 
Your mother's brother is your uncle.
Your mother's uncle is your great uncle.
Many genealogy programs include a feature that calculates relationships between family members. The relationship between a grandmother's brother and a her grandchild is "grand uncle" or "great uncle." Learn more at 23andme.com.