Like, in a perfect world, I wouldn't need to do this, but I consider it an escape rope of sorts, so that I don't feel like I wasted ALL the time coming up with so much backstory for characters who aren't even mine, who nevertheless were never given enough when my mind craved more.

It IS nice to give By Dark/Brother Disease and Brother Mend some actual meaningful interaction outside of their own family, or just being mentioned in lore dumps for other things. The way both of them work - and have always worked - lend very well to a serial immortality survival 'horror' narrative