Your great nieces or newfews
Your niece's children are your great nieces and great nephews.
Nieces and nephews are the children of your siblings.
Nephews and/or Nieces
They are your nieces and nephews and not your cousins
Your daughters are great aunties if their nieces have kids. Your daughters kids are your grand children.
Nephews and nieces
Your borther-in-law's children are your nieces and nephews. You are their uncle or aunt, as is your spouse.
Cousins
The children of your husband's siblings are your nieces or nephews. Their children are your great-nieces or great-nephews.
Your brother's wife's kids would be your nieces or nephews.
They are your grand-nephews and grand-nieces or great-nephews and great-nieces.
Your brother's child's children are your grandnieces or grandnephews.