This child reclines somewhat against the photographer’s chair. Children before the age of 6 months can still have difficulty holding themselves up due to development of their back and neck muscles. Does this child look to be under that age? To me, no. This may have simply been an “artistic” pose the photographer chose. And no, I do not think this is a post mortem.
Note the writing on the tintype – Sister Annas ol? Sister Ann as ol?
I don’t know what that meant but I suspect it was intended to say her age – ol with a ‘d’ on the end is old. What could the “as” looking text translate to?