If your great uncle married into your family some of his nieces and nephews are your aunts or uncles (or your parent) and some are not related to you at all.
If your great uncle is the brother of one of your grandparents, his wife is your great aunt but their nephews and nieces onher side are not related to you. Those nieces and nephews (and their children the great nieces and great nephews) have no ancestor who is also one of yourancestors.
They are your nieces and nephews.
Jane Austen never married and had no children.
Your brother's wife's kids would be your nieces or nephews.
No. He had a brother patrick who married but had no children.
Nieces and nephews are the children of your siblings.
Nephews and/or Nieces
Harper Lee does not have any known nieces or nephews as she never married and had no children. She led a very private life and did not have any immediate family members.
You are not related to all of your brother-in-law's family, but you are related to some of them. You are related to your brother-in-law. You are related to his wife, your sister. If he and your sister have children, you are related to them - they would be your nieces and nephews. But you are not related to your brother-in-law's siblings, cousins, parents, aunts, uncles, grandparents, etc.
They are your nieces and nephews and not your cousins
Nieces are the daughters of your brothers and sisters. Nephews are the sons of your brothers and sisters.
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.