Second cousins.
He is their Uncle and they are nieces and nephews.
Your brother-in-law's children are your nephews and nieces.
Your brother's children would be your nieces and 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 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.
Your sister's husband is your children's uncle. They are his nieces and nephews.