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.
He calls his sons brother son also
Your brother-in-law's son is your nephew
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