Your sister's son is your nephew and you are their aunt or uncle. Their baby would be your great nephew. Some use the term grandnephew, but great is more accurate.
That would be your nephew.
nephew
nephew
Your cousin.
nephew!
We would infact, cll our sisters son your nephew and they would call you either auntie or uncle depending on your sex.
Our cousins are our mothers sisters son :)
My sister's son is my nephew.
My sister's son is my nephew.
If your sister had a son, then he is your nephew.
You would call your sister's son your nefu and her daughter neice.
Your father's sister is your aunt. Her son is your first cousin. Her grandson is your first cousin, once removed.
Your son would be the baby's uncle. Nobody would put such a fine point on the relationship to call your son the "Step-uncle" or "Half-uncle".
He is a baby, not yet hatched. He also calls him a "fry", a baby fish.