You Aunt is you grandson's Great-Great Aunt. Your grandson is your Aunt's Great-Great Nephew.
Your great aunt's grandson is your second cousin.
Your aunt's great grandson is your first cousin twice removed.
Your sister would be your grandson's great aunt.
Your great aunt's great grandson is your second cousin, once removed.
Your sister is your grandson's great aunt. He is her great nephew.
second cousin
Aunt and nephew
Your father and your aunt are siblings (brother and sister) The grandchildren of siblings are second cousins to each other.
Your brother's grandson is your great nephew and you are his great uncle if you are a man, or great aunt if you are a woman.
3rd cousins
Your great aunt is the sister of your grandparent Her son's son is her grandson. Her grandson and you share a great grandparent and thus are Second Cousins to each other.
You and your great great aunt's great grandson are third cousins.