Your sister's son is your nephew.
Stepson; Son in law; Brother?
Grandaughter.
Nephew
Your son's wife is your daughter-in-law. Her brother is your son's brother-in-law, but the English language has no word for a relationship between you and him. You are not considered to be related to him.
daughter-in-law
Son-In-Law
Yes the had a son name Sean Lennon. Yoko has a daughter from a previous relationship and John has a son from a previous relationship.
Your sister's daughter and your son are first cousins to each other.
Neice or nephew no dummy its nephew
Your son's brother in law is likely your daughter's husband. If that's the case he's your son in law.
If her son is from a previous relationship, then he would be your step-son.