Your niece's son is your grand-nephew. You are the child's grand aunt, or grand uncle (whichever applies).
Your great niece's son is your great great nephew.
Your niece's son's son is her grandson. He is your great great nephew and you are his great great aunt or great great uncle, depending on your gender.
Second Cousin
Great-niece.
They would be great nieces or great nephews.
Your grandfather's niece is a first cousin of one of your parents. Her son is your second cousin.
They are your great nephews and/or great nieces. Alternately, some people say grand-nephews, grand-nieces.
Your niece's son is your grand nephew, and you are his great aunt.
Your borther-in-law's children are your nieces and nephews. You are their uncle or aunt, as is your spouse.
You are her great-great Aunt (or great grand Aunt, depending upon your preferred nomenclature). She is your great great niece.
Your aunt's grandchildren would be your first cousins once removed, as the your aunt's children are your first cousins.
Your nephew's son is your great nephew.
Great aunty