The son of your aunt and uncle is your first cousin.
Your niece's son is your great nephew. You are his great aunt or great uncle.
You are your nephew's uncle, or aunt, and his son's great uncle, or great aunt. Your nephew's son is your great nephew.
You would be his Great Uncle or Great Aunt He in turn would be your Great Nephew.
Your sister's son is your nephew. You are his aunt or uncle.
You are their great-uncle/aunt.
Your brother's son is your nephew. If you are female you are his aunt. If you are male you are his uncle.
Your aunt's child, son or daughter, is your First Cousin. The same is true for your uncle's children.
Your sister's (or brother's) son is your nephew. You are his aunt or uncle.
Your first cousin must be the son, or daughter, of your aunt. That is how the relationship is defined.A cousin is the child of your aunt and uncle. An aunt or uncle is the sister or brother of your parent, or the spouse of such a sister or brother.
you are still are considered an aunt or uncle. But, if you are to introduce them to someone outside the family then you are to be considered their great aunt / uncle
Your brother's (or your sister's) son is your nephew and you are his aunt if you are a female or his uncle if you are a male.
Your aunt's child, son or daughter, is your First Cousin. The same is true for your uncle's children.