step niece
nephew
nephew!
Your cousin.
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.
Step son or not regarding your mother's cousin that would make him your third cousin.
step sisters
They could be said to be your step-grandchildren, but there is no real connection to you as such in the genealogical sense, or even to your son. So some may seem them as your step grandchildren whereas others would see them as just being your son's step children and nothing to you.
You would call your sister's son your nefu and her daughter neice.