Your first cousin's grandson is your first cousin twice removed.
Second cousins.
Your grandson and your brother's grandson are second cousins to each other.
Your niece and one of your grandson's parents are first cousins. That makes your niece a first cousin once removed of your grandson.
Your grandson and your first cousin are first cousins, twice removed. Your grandson and your second cousin are second cousins, twice removed. and so forth.
Your niece's child and your grandson's child (your great grandchild) are second cousins, once removed, to each other.
First cousins
First cousins
Your grand niece and your grandson are second cousins to each other
Your sister's child and your grandson are first cousins once removed.
Your grandson and your first cousin are first cousins, twice removed.
Your first cousin's grandson son is your grandson's third cousin. Your second cousin's grandson is your grandson's fourth cousin. Etc., etc.
Your second cousin and your grandson are second cousins, twice removed.