I agree it is ambiguous, however usually you don't separate parentheticals by commas inside a comma-separated list, so the "more likely" reading is the one without the parenthetical.

An unambiguous version that doesn't diminish the joke would be "Lord Byron, John Polidori, Mary Shelley, and her husband". But alas it's difficult for writers to find ambiguities in texts that they write (that's what editors are for).