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
Your husband's sister's kids are referred to as your nieces and nephews by marriage. More specifically, you can call them your "nieces" if they are girls and "nephews" if they are boys.
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.
You can refer to your sister's children as your nieces and nephews. If your sister has daughters, they are your nieces, and if she has sons, they are your nephews. Collectively, you can call them your nieces and nephews or simply refer to them as your sister's kids.
Your nephews' children would be your children's cousins. They share a common grandparent, as their parents (your nephews) are siblings to your children’s parents. This familial relationship creates a bond among them, typically fostering a close connection within the extended family.
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