How is your niece's cousin related to you?

There are three possible relationships between your niece's first cousin and you. For second cousins and more distant relations, the possibilities expand further.

Your niece is the child of your brothers or sisters. Her first cousins would include all the children of any other siblings in your family. Therefore her first cousins could be your sons or daughters, or your other nieces and nephews.

Your children are your niece's first cousins because you and one of her parents are siblings. But your niece has two parents. The one who is not your sibling may also have brothers and sisters whose children will also be first cousins to your niece. However, those first cousins are not related to you at all.