I think this is one of the creepiest Post-mortem photographs floating around out there.
These photographs cost far more to have taken than normal portraits, and were highly desired in the Victorian era. They gained popularity in the US, where there was a rising mortality rate among the homesteaders and other people living in the harsh conditions of the western half of the country. Other causes of high mortality in the US in the 19th century, beyond the hazards of the wilderness out west, were of course the Civil War, railroad accidents, and industrial related accidents. Death was highly present in American Victorian consciousness.
The presence of death was then coupled with the invention and rise of the photograph. Photography, like reflections in a mirror, duplicated the image of whoever was in its path. This duplicate had been seen in various forms of folklore as being a window, or even an outward manifestation, of the soul. Photographs differed from mirrors in a very significant way, though: photographs kept the image even after the person was long gone.
I think it makes perfect sense within this context that a photograph would be a coveted object, even, or especially, a post-mortem one.
The dead were often posed with the living, as in this photograph, perhaps as an attempt to conceal even further the fact that they were dead.
In case you couldn’t tell, the girl in the middle is the one in this photo who is not alive.