Your niece's grandson is are your great-grandnephew.
Your grandson and your brother's grandson are second cousins to each other.
Your granddaughters and son have the relationship of nieces and uncle. Your granddaughters being his nieces and your son being their uncle.
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 sister would be your grandson's great aunt.
Your daughters are great aunties if their nieces have kids. Your daughters kids are your grand children.
He is their Uncle and they are nieces and nephews.
Your great aunt's grandson is your second cousin.
Your sister is your grandson's great aunt. He is her great nephew.
You Aunt is you grandson's Great-Great Aunt. Your grandson is your Aunt's Great-Great Nephew.
Your grandson is your brother-in-law's great nephew, and your brother-in-law is your grandson's great uncle.
Second cousins.
Your grand niece and your grandson are second cousins to each other