He would be his great great great uncle
Your great uncle's grandchild is your second cousin.
You are their great aunt/uncle.
2nd uncle I believe
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.
You would be his Great Uncle and he would be your Great Nephew.
Your uncle's child is your first cousin. His grandchild is your first cousin, once removed. You are also that grandchild's first cousin, once removed.
Your borther-in-law's children are your nieces and nephews. You are their uncle or aunt, as is your spouse.
Your are a great uncle. I mean, I'm sure you're a wonderful uncle! But great uncle is your relationship. The child is your wife's sister's great grandchild, right? So you are great uncle.
You and your uncle's great grandchild are first cousin, twice removed.
Second cousins.
Huey, Dewey, and Louie Duck were the three nephews of Uncle Scrooge.
You and your uncle's child are first cousins to each other. You and your uncle's grandchild are first cousins, once removed, to each other.