Everybody seems to miss the fact that..
Spoilers:
They never left the influence of the mirror. After the dad dies in the 'past' events the daughter makes a promise with the son to come back and "kill it". She spends a great deal of effort to track the mirror so she can go back to it.
Notice how in all the examples she gave about the mirror, the mirror always won (including the guy running into traffic). After the dad dies, there are till 2 survivors. The police would be there any minute because several shots were fired. The mirror still had a job to finish, and it made Kaylie come back so it could finish the job.
Although I agree this is not a masterpeice by far, there are no plotholes. THey did not stand a chance, the power of the mirror is practically unlimited. If you think that fact is a reason to dislike the movie, then I understand completely. But don't come saying about how bad the decisions of the characters are.
Also, I thought the cinematography, acting, directing, etc was worth atleast a 6. Maybe not a movie I would recommend, but sometimes I'm not in the mood for a 4 hour masterpiece, sometimes I just want an entertaining 105min and this gave it to me.