Your niece's children are your great nieces and great nephews, and you are their great uncle.
Your uncle could be the father of your niece, and thus the grandfather of your niece's children.
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.
Uncle if you are male and aunt if you are female.
You can date anyone you like, even a sibling. Incest or other undesirable relationships develop only if your dating moves on to sexual activity.Many people who are related to your uncle are also related to you, but only some of them will be so closely related as to make a sexual relationship forbidden. Those people who are related to your uncle because they are the nieces, nephews, great nieces, great nephews, etc., of his wife are not related to you at all.
Nancy
Your cousin's child is your first cousin, once removed. Then when that child grows up and has a child, it will be your first cousin, twice removed. Now, if you have a child, and your cousin has a child, your children will be second cousins to each other. Then their children will be third 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.
You would refer to them as your nieces, thought they are formally your wife's nieces. And they would normally call you uncle.
Your daughters are great aunties if their nieces have kids. Your daughters kids are your grand children.
Your granddaughters and son have the relationship of nieces and uncle. Your granddaughters being his nieces and your son being their uncle.
depands if on your side or hers
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.
Call the great nephew Joe and the great aunt Pamela. If Pamela is the sister of Joe's grandmother or grandfather, then Joe and Pamela are "blood relatives." However, Pamela could be the wife of a brother of one of Joe's grandmother's or grandfathers and still be Joe's great aunt. In that case, Pam and Joe are not "blood relatives."
He is their Uncle and they are nieces and nephews.
Your grandfather's brothers would be the uncles of your parent. That would make them your great uncle.
Your uncle through marriage is the husband of your aunt, the sister of your father or mother. All of your aunt's nieces (the daughters of her siblings) are also his nieces. So you (if your are female) and our sisters are nieces of your uncle through marriage, as are the other daughters of the siblings of your parents.Also, your uncle through marriage has nieces who are the daughters of his bothers and sisters. They are not related to you because you do not share any ancestor with them.
The children of your husband's siblings are your nieces or nephews. Their children are your great-nieces or great-nephews.