What is the relationship to your younger brothers wife's sisters daughter to our son?
Your younger brother's wife's sister's daughter would be your son's cousin.
What am I to my sisters son son?
Uncle or Auntie.
What relationship is son's son to sisters daughter?
Your sister's son is your nephew.
Your son's daughter is your granddaughter.
Your son and your sister's son are first cousins to each
other.
Your son's daughter and your sister's son are first cousins once
removed to each other.
Your grandchild and your sister's grandshild are second cousins
to each other.
What relationship to you is your wife's son?
If her son is from a previous relationship, then he would be
your step-son.