Your great niece's children are your great great nephews (the boys) and great great nieces (the girls). You are their great great uncle (if you are male) or great great aunt (if you are female).
Your niece's children are your great nieces and great nephews.
Your great nieces or newfews
They are your grand-nephews and grand-nieces or great-nephews and great-nieces.
The children of your husband's siblings are your nieces or nephews. Their children are your great-nieces or great-nephews.
Your great niece's children are your great great nephews (the boys) and great great nieces (the girls). You are their great great uncle (if you are male) or great great aunt (if you are female).
Your brother's child's children are your grandnieces or grandnephews.
Some would call your nephew's children your grand nephews and grand nieces, others would call then your great nephews and great nieces.
Your daughters are great aunties if their nieces have kids. Your daughters kids are your grand children.
Your borther-in-law's children are your nieces and nephews. You are their uncle or aunt, as is your spouse.
They would be great nieces or great nephews.
yes
You are Great Aunt to your niece's child. The child is your great niece or great nephew.