Your uncle's son is your first cousin.
It means you have (or had) at least two uncles and at least two great-uncles, and that is all it means.
your uncles son is your cousin
His great uncle
Your cousin in law
great uncle
Your mother's brother is your uncle.The children of your aunts and uncles are your first cousins.
your cousin
Your uncle/aunt's son/daughter is an adult and you are still small, your uncle/aunt's daughter had a child, and that child will call you uncle. Uncles/aunts are your dad/mom's siblings.
No, "uncles" does not have an apostrophe. An apostrophe is used to indicate possession or to form contractions, but in the case of "uncles," it is simply a plural noun.
Your brother's daughter's daughter is your great niece.
Yes. Any son or daughter of Zeus would be her half-sibling. Any son or daughter of Poseidon or Hades would be her cousin as Poseidon and Hades are her uncles.
Your uncle's grandchild would be your second cousin because his grown son or daughter would be your first cousin.