Your great aunt's son is first cousin to one of your parents, and first cousin once removed to you.
That's easy cousins.
A nephew is a cousin to his aunt's son.
Your great aunt's son is first cousin to one of your parents, and first cousin once removed to you.
Your sister's son is your nephew. You are his aunt if you are a woman, or his uncle if you are a man.
Your sister's son is your nephew. You are his aunt, if you are female, or his uncle, if you are male.
Your father's sister is your aunt and her son is your first cousin.
Your son and your aunt's son are first cousins once removed to each other.
He is the nephew and you are the uncle or aunt, depending upon your gender.
Your mother's aunt's husband's sister is your mother's aunts sister-in-law but is not related to you. Her son is not related to you either.
You are the great aunt/uncle of your sister's son's daughter, and your sister's son's daughter is your great niece.
Your son and your mother's sister are in a "Great Nephew/Great Aunt" relationship.
Your niece's son's son is her grandson. He is your great great nephew and you are his great great aunt or great great uncle, depending on your gender.