They are your nieces and nephews and not your cousins
Yes
Nieces and nephews are the children of your siblings.
Nephews and/or Nieces
Your niece's children are your great nieces and great nephews.
Your borther-in-law's children are your nieces and nephews. You are their uncle or aunt, as is your spouse.
They are your grand-nephews and grand-nieces or great-nephews and great-nieces.
The children of your husband's siblings are your nieces or nephews. Their children are your great-nieces or great-nephews.
Mark Twain's nieces and nephews were the children of his siblings, which included nieces Ida, Susy, and Clara, and nephews Samuel, Frank, and Willie.
Nephews and nieces
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.
Rachel Carson did not have any biological children, so she did not have any nieces or nephews.
Your brother's wife's kids would be your nieces or nephews.
If it is a boy then it is your nephew. If it is a girl then it is your niece.