Your first cousin and your son are first cousins, once removed, to each other.
Your mother's sister's son is your first cousin.
Your first cousin's sister is also your first cousin. Your second cousin's sister is also your second cousin. And so forth. The child (son or daughter) of your first cousin is your first cousin, once removed. The child (son or daughter) of your second cousin is your second cousin, once removed. And so forth.
Your first cousin's nephew is the son of your first cousin's sister or brother. That sister or brother is also your first cousin, and the son is your first cousin, once removed. So your first cousin's nephew is your first cousin, once removed. Similarly, your second cousin's nephew is your second cousin, once removed.
Your First Cousin and your children are related as First Cousins once removed.
Your great aunt's son is your parent's first cousin and your first cousin, once removed.
The first cousin of your dad's first cousin is also your your dad's first cousin or is not related to him or to you. The children of two first cousins are second cousins to each other. So the son of the first cousin of your dad's first cousin is either your second cousin, or is not related to you at all.
Your father's sister is your aunt. Her son is your first cousin.
Your first cousin's son is your first cousin once removed. The son's wife is not related to you. 1st cousin once removed by marriage is your relation to her, even though she isn't technically a blood relative, which really shouldn't matter, give her a hug and call her cousin.
Your mother's first cousin's son is your second cousin. Her first cousin's son's son (her first cousin's grandson) is your second cousin, once removed.
Your great aunt's son is your first cousin once removed. His daughter is your second cousin.
Your daughter's paternal first cousin is your first cousin, or your husband's first cousin. Your first cousin's father is your uncle. Your uncle's wife is your aunt. The son of your aunt and uncle is your first cousin and your daughter's first cousin, once removed. If your uncle's wife has a son fro a previous marriage, that son is not related to you or to your daughter, although you may find is socially or personally desirable to treat him as if he were your first cousin.
Your father's brother is your uncle. Your uncle's son is your first cousin and your children's first cousin once removed.If your first cousin has a child, that child and your children are second cousins.