Uncle or Auntie.
Your sister's son is your nephew and you are his aunt (if you are female) or his uncle (if you are male.
Your sister's son is your nephew.
No, making your son wear his sisters pants is not a good punishment.
Cousin
He is your cousin
nephew
Our cousins are our mothers sisters son :)
Your sister's daughter's son would be your grandnephew.
A siblings son is always called a nephew
My sister's son is my nephew.
nephew!
cousin