my interpretation
the family has been dead for a week. one day, nicole kidman goes crazy because of the stress of the disease her kids have, no husband, etc. so she kills them and the servants disappear because they are now in separate worlds and they don't sense them because they probably left after discovering the body. she tries to send the mail, and of course the mail man doesn't come. the new servants come because they discover new ghosts, and slowly try to ease her into the reality that she is a ghost. the fog is only for the ghosts - "it's never lasted this long before". the husband is dead, and when he comes back to say goodbye, anna tells him that the mom attacked her. since they can see him, they are dead, and he puts two and two together and realizes that she killed them.
great film, just saw it today