Children of your siblings are nieces and nephews. Children of your spouse's siblings are still called nieces and nephews, but you are not related to them by blood.
Your mother's sister's children are your 1st cousins.
Your sister-in-law is either the wife of your brother or the sister of your spouse. The children of your brother's wife are your nieces and nephews, since they are also the children of your brother. In some cases they may be your sister-in-law's children from a previous marriage. Then they are not related to you at all. However, if they are very young, it may be convenient to call them your nieces and nephews anyway. The children of your spouse's sister are always your nieces and nephews.
Nieces and nephews are the children of your siblings.
Nephews and/or Nieces
If your great uncle is the brother of one of your grandparents, his wife is your great aunt but their nephews and nieces onher side are not related to you. Those nieces and nephews (and their children the great nieces and great nephews) have no ancestor who is also one of yourancestors.
They are your nieces and nephews and not your cousins
Nephews and nieces
Your borther-in-law's children are your nieces and nephews. You are their uncle or aunt, as is your spouse.
The children of your husband's siblings are your nieces or nephews. Their children are your great-nieces or great-nephews.
They are your grand-nephews and grand-nieces or great-nephews and great-nieces.
No she does not have any children. She has several neices and nephews, but no children.