Your son and your niece are first cousins to each other. Your son and your niece's son are first cousins, once removed.
You and your first cousin's son are first cousins, once removed.
Your son and your niece are first cousins. Her daughter and your son are first cousins, once removed.
Your niece and your son are second cousins. Your niece's child and your son are second cousins once removed.
Your daughter's son's son is your great grandson. Your daughter and your sister's son are first cousins. Your great grandson and your sister's son are first cousins, twice removed. Your great grandson and your sister's great grandchild are third cousins.
your son and your brother's son will be cousins
your son and your brother's son will be cousins
Your daughter and your first cousin's son are second cousins to each other.
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.
your son and your brother's son will be cousins
Your daughter and your sister's son are first cousins to each other.
Your son and your aunt's child are first cousins, so your son and your aunts grandson (the son of your aunt's child) are first cousins, once removed.