I can't believe a game like this doesn't have the incest tag.
Or is it the usual, "landlord and roommate" kind of thing?
Think the relationship is assume to be those complex as fuck "Protag mother died/divorced, his father remarried to a single mother with a daughter, then the father pass away." ordeal.
So not incest, since he already call the so call mother by name, which probably means she is step mom.
(Don't know about Japanese culture, but than again, culture accuracy doesn't seem very on point anyway.)
The basically "Take place in America version" of this game known as "Another Chance" kind of use the same concept when dealing with the sister and mother.
In that game the protag also call his "mother" by name...which I am just gonna assume that is stepmom.
(Oh and also people are generally nicer in that game then in this one...which is doubly weird since that game you play as a loser in an American highschool...)