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 Uncle.
I call my mother mom
Your mother's mother is your grandmother.
don't do it - if your brother has already messed up his credit - he will do it again and risk yours as well
He is your uncle, in most families he would be Uncle (Jack, Fred or what ever his first name is).
In Farsi, you would say "amoo" for mother's brother.
brother
You could call them as a brother/sister or as a cousin... If you're asking for yourself then good luck with that family.
Sup granny
yours
His relationship to you is brother-in-law.
uncle