From what I can tell from an admittedly quick glance around, experts at the National Media Museum have been dubious, while some members of the Brontë Society are aiding in research to (possibly) authenticate it. The photo currently is believed to be a ambrotype copy of a daguerreotype. The photographer who may have copied it is John Stewart of Pau, France & London, England. If these are the Brontë sisters, the original photo must have been taken before Emily died in 1848.
Here's an interesting website that 1) compares the women's faces in this photo with known portraits and written descriptions of each of the three Brontë sisters made during their lifetimes and 2) constructs a possible history for the photo based on the evidence it provides. Intriguing!
What do you think?