How am i related related to my sisters great granddaughter?

You are both second cousins once removed. The common ancestors of you and of the your great grandmother's sister's daughter's son are one of your sixteen pairs of great great grandparents, who are also one of the eight pairs of his great grandparents. Children of your parents' siblings ar your first cousins; of your grandparents' siblings, are your second cousins; of your great grandparents' siblings are your third cousins. Your great grandmother's sister's daughter's son is a grandson of your great grandmother's sister, and he is the second cousin by birth of one of your parents. You are therefore at one further remove than he is from the common ancestors of you both, and therfore second cousin once removed. (Of course, I am considering only people who are not unrelated in other ways, which could complicate matters--there was a popular song in the middle of the last century which had the title of "I am my own grandpa'.