The child of your parent's brother or sister is your first cousin. However, your first cousin's child is NOT your second cousin, but your first cousin ONCE REMOVED. The child of your first cousin once removed is your first cousin TWICE removed, and HIS child is your first cousin THREE times removed.
Your second cousin is your grandparent's brother's or sister's grandchild. That second cousin's child is your second cousin once removed, and his child your second cousin twice removed, and his child your second cousin thrice removed, and so on.
And your third cousin? It's your great-grandparent's brother's or sister's great-grandchild. The third cousin's child is your third cousin once removed, and his child your third cousin twice removed.
The grandchild of your brother or sister is your grandnephew or grandniece.
The sister or brother of your grandparent is your great-aunt or great-uncle.
The sister or brother of your great-grandparent is your great-grand-aunt or great-grand-uncle.
