Your sister is your grandson's great aunt. He is her great nephew.
Your sister would be your grandson's great aunt.
Your sister's child and your grandson are first cousins once removed.
nothing
Your great-grandmother's sister's grandson is your second cousin once removed.
Your grandson and your sister's grandson are second cousins to each other.
Your second cousin once-removed.
2nd cousin once removed
Your daughter's son's son is your great grandson. Your daughter and your sister's son are first cousins. Your great grandson and your sister's son are first cousins, twice removed. Your great grandson and your sister's great grandchild are third cousins.
Your granddaughter and your sister's grandson are second cousins.
Your father and your aunt are siblings (brother and sister) The grandchildren of siblings are second cousins to each other.
Your grandmother's sister's grandson is your second cousin.
Your grandson is your sister's great nephew and she is his great aunt.