Well, they are related but a name has not been given to that yet! :)
You are an aunt or uncle (depending on your sex) to your nephew, and a great aunt or great uncle to his children.
Your nephew's child is your great nephew (if a boy) or great niece (if a girl). You are his or her great aunt
Your children and your nephews and nieces are first cousins to each other.
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.
Great nephew
Nephews are the sons of your brother or sister. Therefore nephews who are not brothers to each other are first cousins to each other.
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.
Second cousins.
Your brother's wife's kids would be your nieces or nephews.
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-in-law's children are your nephews and nieces.