Want this question answered?
They are your grand-nephews and grand-nieces or great-nephews and great-nieces.
They are your great nephews and/or great nieces. Alternately, some people say grand-nephews, grand-nieces.
Some would call your nephew's children your grand nephews and grand nieces, others would call then your great nephews and great nieces.
Your nephew's children are your great nephews and great nieces (or grand nephews and grand nieces).
Your niece's children are your great nieces and great nephews.
You would be their great-granduncle or great grand aunt, and they are your great-great nieces or great-great nephews.
The children of your husband's siblings are your nieces or nephews. Their children are your great-nieces or great-nephews.
Your borther-in-law's children are your nieces and nephews. You are their uncle or aunt, as is your spouse.
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.
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."
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.
They would be great nieces or great nephews.