Q. Where does the second comma belong in this phrase: my cousin, Joseph’s son? Is it my cousin, Joseph’s, son? But you wouldn’t say “my cousin son.” Rather it should be “my cousin’s son.”
A. Because commas are not helpful in such constructions, don’t use them: my cousin Joseph’s son. If you worry that commas mean that you have only one cousin, whereas you actually have a passel of them, you can rephrase instead: the son of my cousin Joseph.