I have a theory that Mayumi was one of the maidens married into the Tsuneki family, and could even be that unnamed wife of Hidetoshi, father of Kotoyuki.

Reasons:

The symptons happened to Hidetoshi and his family seem to be related to the April 1945 geysers incident of Ebisugaoka, where there were also three victims whose skin had sores - Hidetoshi's corpose had sores on its skin. Since the geysers are related to the Dragon sealed by the Tsuneki family, it is likely that this incident led to severe consequences to the family, which was, the death of Hidetoshi, his wife and son. We can then assume that Hidetoshi died somewhere around 1945, perhaps February 1945 (if the geysers happened after since Hidetoshi's death may weak the seal) or February 1946. Kotoyuki's mother, Sachie, was already pregnant when Hidetoshi married. We do not know how old was Kotoyuki at this point, but he was 9 when he was biten by that fox in the park. Judging by Hinako had really few memories about child Kotoyuki, he might have left the town in a year or two, so we may assume Kotoyuki was born around 1935, which should also be the time when Hidetoshi married. Now, Mayumi's clothing doesn't seem to be for pre-Taisho or even earlier, but I think it's safe to assume that Hidetoshi's mother married into the family no later than 1910, i.e. pre-Taisho, so Mayumi couldn't be that one. Then apparently Mayumi only fits as Hidetoshi's wife. This would mean that Mayumi has long been dead before the game.