Well, they are related but a name has not been given to that yet! :)
Your sister's husband is your children's uncle. They are his nieces and nephews.
you are their gran-uncle or gran-aunt depending on your gender. they are your gran nieces and gran nephews depending on their gender.
The children of your husband's siblings are your nieces or nephews. Their children are your great-nieces or great-nephews.
Nephews are the sons of your brother or sister. Therefore nephews who are not brothers to each other are first cousins to each other.
Great nephew
They are your nieces and nephews.
Your brother's children would be called your nieces or nephews.
He is their Uncle and they are nieces and 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
Second cousins.
nece and nefu i spelled it wrong though sorry if someone could update this better Your daughters are his nieces by marriage and your sons are his nephews by marriage.
Your brother's children would be your nieces and nephews.