Responses
Heavy spoilers for the ending if I remember correctly
The fantasy game is actually a simulation in which he ml tried to conserve his sister's spirit or smth in there cuz she was either in a coma or died and ml couldn't live without his last(?) family member. So he gave up on reality and decided to live in that made-up world with her. He eventually forgot about reality, the sister didn't, and the fl, I think, was chosen by the stuff or his friends to make him want to leave that place and accept his sister's death. Their love story was like a bonus.
At first I thought it will be a stupid story, but the plot twist was actually nice