Från NessieMackenzie ,

we know that Mackenzie Foy is playing Renesmee Cullen in Breaking Dawn, but they still needed another girl with Mackenzie’s body structure for the stand-in and photo-double. According to IMDb, Haley Duryea is the lucky girl who gets to take on the stand-in job!

A stand-in is a person who has the same physical properties of a particular actor, and takes their place during the lengthy setup of a scene. This allows the actor to prepare for the filming itself. Contrast with stunt double and body double.

It doesn’t really matter if she doesn’t look like Mackenzie, they only need to have the same body structure. Haley seems to be older than Mackenzie, so we can guess maybe she’s being used for theBreaking Dawn stills, as an older version of Renesmee (with Mackenzie’s face morphed onto Haley’s face). Or maybe they’re just using her as a stand-in Renesmee.