The sister of your great great great great grandfather is your great great great great great aunt. Her great great grandchild is the fourth cousin of your grandparent and your fourth cousin, twice...
Your great uncle's child is your parent's first cousin and his grandchild is your second cousin. His great grand child is your second cousin, once removed, and his great great grandchild is your...
Your great-n uncle is either the brother of your great-(n-1) grandparent, OR the wife of the sister of your great-(n-1) grandparent, where 'n' represents the number of 'greats', e.g. your great great...