Short answer: They would be your niece and nephew.
Long answer:
Your sister-in-law is either your sister-in-law because she is married to your brother (or, in same-sex marriages, your sister) or because you are married to her brother (or sister in same-sex marriages). If it is the former and she married your sibling, then her children would be your niece and nephew by blood--you share the same gene pool--happy swimming). Or, if it is the latter, and you married her sibling, then they are your niece and nephew by marriage and you do not share the same gene pool--unless there is another familial connection. Either way, you still have to buy them Christmas and birthday presents!
your sister's daughter= your niece
your sister's son= your nephew
Your sister's son or daughter would be your nephew or niece respectively.
Niece and nephew
You would call your sister's son your nefu and her daughter neice.
If she is your sisters daughter then she is your neice :)
She is your niece.
what would my sister's son's daughter be to me? Your sisters son is of course your nephew,You are his Aunt... Your Nephews daughter would be your great-neice,you would be her great-Aunt.
niece
Niece.
Neice
She is your niece.
You would call your daughter's son by his name ex: Hi Jack.
Your sister's daughter's son would be your grandnephew.
His name.
Your daughter is your sister's niece. Your daughter's son is her great nephew.