Stay FAR FAR away from them both!
Assuming that you are a female and that your mother actually has a brother, then yes, you are your mother's brother's niece.
You and your mother's niece are first cousins to each other, so your and the niece's daughter are fist cousins, once removed.
Your niece would be your mothers granddaughter.
You are your mother's aunt's great niece.
Your mother's niece is your first cousin. Your first cousin's daughter is your first cousin, once removed.
You are your niece's aunt, if you are a woman.Any of your sisters who are not your niece's mother are also her aunts.The sisters of your niece's parent who is not your sibling are also her aunts.
Niece. Your mother has children, you and your sister. So your mother's daughter is your sister. When she has a daughter, this is your niece.
Sister.
A mother.
You son and your niece are first cousins. Your son and your niece's son are first cousins, once removed.
Your Mother would be your Neice's "Great Aunty"
Your grand mother's niece is your parent's first cousin. Her children are your second cousins.