Photography was expensive back in its infancy but much cheaper than a painting. So people would do postmortem photos of loved ones to keep the memories alive. In that particular photo, they wanted the kids in the photo but due to long exposure time, the mother would sit, covered, to keep the living kids still for the duration of the photo.
So yes, it’s a valid example of postmortem photography. It’s actually a modern equivalent of the death mask of ancient times.
Early photographic images had to be exposed for a very long time in order to catch them on film. Because children were unlikely to keep still for long enough their mothers were often present but hidden (not very well in this case) in order to try and keep their children still.