Your great nieces or newfews
Your niece's children are your great nieces and great nephews.
Nephews and/or Nieces
Nieces and nephews are the children of your siblings.
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.
if they are the children of your brother, they would be your nephews and nieces - if they were her children before she married your brother, then step-nephew and step-nieces
They are your grand-nephews and grand-nieces or great-nephews and great-nieces.
Rachel Carson did not have any biological children, so she did not have any nieces or nephews.