Your mother's brother is you uncle and your daughter's great uncle. Your daughter is his great niece.
brother
your brother's daughter is you niece
uncle
brother's daughter relation
Your brother's daughter is your niece. Her daughter is your great niece.
You're not related.
Your brother's son or daughter is your nephew or niece respectively.
Mother's grandson as regards her daughter is either her (daughter's) son or her (daughter's) nephew.
granduncle - grandnephews/grandnieces
Your grandfather's brother's daughter is your first cousin, once removed.
I guess you will be her uncle. Consider this case.. You have a brother and he has a daughter.. Your daughter will be a sister in law to your brother's daughter.
3rd cousin I think:-)