They share a common Grandparent. That makes them first cousins.
Your sister's son is your nephew His sons are your grandnephews, and his daughters are your grandnieces.
Your brother-in-law's son is your nephew
He calls his sons brother son also
Your brother's son is your nephew. Therefore, your brother's son’s son would be your great-nephew. In summary, your brother's son son is your great-nephew.
Your son's children are your grandchildren.
Well, your cousin's brother is also your cousin, and a cousin's son is also your cousin.
There is no genealogical relationship. He is just your son-in-law's brother.
Your brother's son or daughter is your nephew or niece respectively.
Your son would be the uncle of his brother's son.
A second cousin.
The sister's son would be the nephew of her brother.
God Brother