A couple things I should've asked earlier: First, are all the faces that appear of the same person or set of persons? Second, has the event been tested in a controlled experiment, whereby the family is moved out of the house and the house is guarded and left empty for a period averaging the time it takes for the faces to completely form?
If this were done, then it might shed some light on the whole thing. There are a lot of chemicals out there that could easily soak into cement ('specially oils) and it could still be possible that they have been faked by the family.
Another thing: I think each of the faces should be documented, and an investigation should be run on each one to see if there were any people matching those descriptions living at the site in the past.
J0N