According to the chart used by courts and most genealogical societies, your brother's daughter's daughter would be your Grand Niece
Reference: http://freepages.genealogy.rootsweb.ancestry.com/~gentutor/chart.html
Your Uncle.
I call my mother mom
Your mother's mother is your grandmother.
He is your uncle, in most families he would be Uncle (Jack, Fred or what ever his first name is).
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.
Mothers brother ( uncle ) = amoo
His relationship to you is brother-in-law.
group of mothers
uncle
Your mother's son is your brother or your half brother.
A cousin brother means , the son of the father or mothers , brother or sister.