Your daughter and your nephew are first cousins. His daughter is your daughter's first cousin, once removed.
Your daughter and your nephew are first cousins. His daughter is your daughter's first cousin, once removed.
Second cousins once removed.
You are their great aunt/uncle.
3rd cousin
2nd uncle I believe
Nephews and nieces
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.
Your borther-in-law's children are your nieces and nephews. You are their uncle or aunt, as is your spouse.
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.