@
Asingh89, @
Tcof and @
virium:
The guy in the hospital was/is his uncle or (if I understood it correctly in chapter 1) the husband / exhusband / widower of his aunt (from the side of the mother's family).
It is stated in chapter 1 "the father of my mother's nephew" which is logically his uncle (also called that way in chapter 1).
Because the "uncle" is referred to as "in-law" and counts as "distant relative" I guess he is not blood-related to the MC but the the husband of one of the MC's aunts.
That uncle had some accident a while after the MC had arrived and has been hospitalized since then.
That is why the MC visits him sometimes in the hospital.
As for what "accident" the uncle had there is no explanation but with the story going on it seems to be sometimes implied that his "accident" has something to do with the occult.
After all the MC also came in contact with the occult meeting his beloved new "onee-san" in his uncle's storage room.
In case the uncle's "accident" is connected to his occult research or occult activities it stands to reason that the mysterious women shown in chapter 19 might also be connected to the occult.
She could either be something like a companion / colleague researcher or practitioner of the occult or she could be a special being like our MC's "onee-san" in a way like being a demon, evil spirit, youkai or so.