To adhere strict scientific logic to the premise is futile-- the appearance of a mirror Earth is a philosophical conceit, not a scientific one, and to get hung up on the exacts of the premise is to miss the point. The underlying problem being addressed is dealt with below in the spoiler text, which absolutely under no circumstances should be revealed unless you've seen the film-- this contains an ENORMOUS SPOILER you will not want to ruin for yourself!
I know there will be no end of interpretations of the final scene. Ultimately, the Mirror-World Marling's appearance must mean that the husband's family did indeed still die in the Mirror Earth, because how else could she have landed her ticket to Earth 1? And thus, not only is it impossible to fix a mistake made on Earth using all tools at our disposal (emotional, physical, etc), even a majestic implementation of the cosmic cannot undo what's been done. There is perhaps some small positive note in the fact that Mirror-World Marling is not as good a person as Earth 1 Marling (she went and didn't give the ticket to the husband), but perhaps she's better and never even tried to get involved in his life? When the final scene appeared, I nodded my head "Of course"-- I hadn't seen it coming, but it made instant sense. However, as the credits rolled, the full implications of the ambiguous final moment hit and I am not ashamed to say I was overcome with emotion. I rarely cry at films, but something about the utter fatalism of not just Marling's actions but all our actions, of the inability to right our past wrongs despite all our best efforts, and even just the pure poetry of such a well-handled ambiguous ending-- all aspects hit hard.