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.
Great aunty
No. Aunty would be your parent's sister or sister-in-law. Second cousin would refer to the relationship your parent's cousin is to you. Similarly, 1st cousin, once removed is the relationship you have to your cousin's child.
Because you are also a cousin.
2nd cousin
Your grandfather's son's son could be:you (if you are a boy)your brotheryour first 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 grandfather's son's son could be:you (if you are a boy)your brotheryour first cousin
A person's relationship to the child of a cousin is the same as that person's relationship to the cousin, with one remove added. If your cousin is your first cousin, then his son is your first cousin, once removed. If your cousin is your second cousin, then his son is your second cousin, once removed. If your cousin is your first cousin, once removed, then his son is your first cousin, twice removed.
Your cousin's son is your cousin, once removed. For example, if your cousin is a second cousin, the son is your second cousin, once removed.
Your son and your first cousin's son are second cousins to each other. Your first cousin's son is also YOUR first cousin, once removed.
Your first cousin's son is your first cousin, once removed. Your second cousin's son is your second cousin, once removed.
Your mother's first cousin's son is your second cousin. Your mother's second cousin's son is your third cousin. And so forth.