Some observations, here's your answers? Spoilers.

This was a well made interesting film, but was it really anything different than Invasion of the Body Snatchers (invasion of the mirror people)? Is it really a deeper film about the loss of personal identity and the "killer instinct" in all of us? Having the family be the central dynamic is definitely an indication that living in a city and having a busy life can tear a family apart, can make you seem cold and distant. The accident is a nice twist but the remarks of the doctors that such a rare case has 80 new instances in recent times is an indication that this is a global takeover (by the mirror people). The woman in the subway, a social misfit, a dropout, can see the evil easily that no one else can see, because we are all a little evil, a little hardened by success and money?

But the scene where Gina's brother's girlfriend is looking in the mirror and then we see the other side, and it's vast and dark and clearly another (jealous?) world, well then, that's actually what the film is about, isn't? An alternate reality inside mirrors, an ancient fear of man, when primitive tribes first saw mirrors they saw other people, not themselves. So, your reflection is so jealous of you and your life, it breaks through the mirror and murders you, not just to kill you, but to take over and live your life, although as a somewhat more angry and cold "you".

So people writing that there is no reason behind the story or no answers given are missing the obvious, there are exact copies of us living behind our reflections in mirrors, mirrors are windows to another world and the copies of us in that dark world are growing jealous and so they break through to kill us and live our lives... there's your answer.