My headcanon is that this bad end occurs if he tries to date a girl after Nanami dies. I don't think the sister would be content if Nanami won, but she probably wouldn't go full yandere over her winning.
Parasocial relationships are completely unhealthy. Only ones aimed at fictional characters are kind of okay; ones that have real people as a target just lead to all kinds of suffering.
I checked the WN and it basically ended with the Sophia battle, with their time travel antics of chapter 43 being described in the first 3 omake chapters. I don't think the 4th and last omake has been adapted, but other than that, the manga really doesn't have anything left to adapt.