you would be the child's great aunty or uncle
Therefore, they would be your great niece or nephew.
Your nephew's child is your great niece or great nephew.
Your nephew's grand child is your great great niece or great great nephew.
Your great nephew's child is your great great nephew or great great niece You are his or her great great aunt or uncle.
Your nephews child is your grand nephew/niece.
Your nephew and your child are first cousins
Great nephew-niece
You are their great aunt/uncle.
3rd cousin
2nd uncle I believe
Nephews and nieces
Your borther-in-law's children are your nieces and nephews. You are their uncle or aunt, as is your spouse.
Your uncle-by-marriage's nephews might be you and your siblings. You are your uncle's nephew or niece.They might be the children of your aunt's siblings (your aunt being his wife), in which case they are your cousins.If they are your uncle's nephews by virtue of being children of his siblings, they are no relation to you at all.
He would be his great great great uncle
They would be great nieces or great nephews.
You would be his Great Uncle and he would be your Great Nephew.
They are your great nephews and/or great nieces. Alternately, some people say grand-nephews, grand-nieces.
that would be your niece or if you want to be fussy then your niece in law!
Second cousins once removed.