It's important that though Alice can see this mirrored room now, she recognizes that she can’t see everything about it because someone else has to make a decision. This highlights how important the idea of choice and free will are within
Twilight’s world: It’s impossible to tell for sure what the future holds because it’s changing all the time based on what choices people make. Generally speaking, seeing this change means that the Cullens are going to have to rework their plans if they’re going to come out on top. But this revelation about how Alice’s ability to see the future works also connects to the novel’s themes about a person’s character depending not on who (or what) they are but instead on what they do.