Your sister's son' son is her grandson. Your sister's grandson is your great nephew. You are his great uncle or great aunt.
My sister's son is my nephew.
YesThe son and his sister = 1 son, 1 sisterThe sons and their sister = many sons, 1 sisterThe son and his sisters = 1 son, many sistersThe sons and their sisters = many sons, many sistersThough in reality this is a strange way to say it. More normal would be "The sons and daughters".
You do you call your sons's son your "grandson" !
Your sister's son is your nephew His sons are your grandnephews, and his daughters are your grandnieces.
cousin
Your sister's son is your nephew His sons are your grandnephews, and his daughters are your grandnieces.
nephews!
First cousin, once removed.
nephew
He calls his sons brother son also
Your cousin.
nephew!