Relationship sequence:
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".
Your sister's son is your nephew His sons are your grandnephews, and his daughters are your grandnieces.
cousin
My sister's son is my nephew.
Your sister's son is your nephew His sons are your grandnephews, and his daughters are your grandnieces.
First cousin, once removed.
Your granddaughter and your sister's grandson are second cousins.
They are second cousins
You are each others cousins.
No, because my father's sister (my aunt) does not have any sons, she only had daughters.
Sons would be plural and son's is possessive. Tickyul
Yes, she did. She had the relationship of a mother that a mother has with her son. As most mothers do.