The English language does not have special terms for a relationship involving an older, or a younger, brother or sister.
Your brother's son is your nephew and you are his uncle, if you are a man, or his aunt, if you are a woman.
We would infact, cll our sisters son your nephew and they would call you either auntie or uncle depending on your sex.
nephew
You would call your sister's son your nefu and her daughter neice.
nephew!
Your cousin.
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.
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.
Your sister's daughter's son would be your grandnephew.
That would be your cousin, honey. Just keep it simple and call them what they are - family. No need to complicate things with all those fancy terms.