An old Dark Sun technique, a variant of which is mentioned on page 236 of the 5E DMG. I will explain:
Classical AD&D might feature a stable of characters, and the DM would tell players at the start of an adventure which of their characters to bring. E.g. "this adventure is for levels 7-9, so bring one of your mid-level guys. Lord Robilard (level 20) stays home." Then they would play that adventure with that PC.
Then in 2nd edition, Dark Sun was envisioned as an extremely deadly environment with high PC turnover, so an additional idea was introduced: instead of just a stable of characters, have a "tree" of replacements available at any given time. In addition to choosing which character to play at the start of the adventure, if your current character died you'd pick another character from your tree and the DM would manipulate the narration to ensure that the new character happened along shortly. (Maybe you were guarding a caravan, and it just so happens that the PCs try to hide from the bad guys in the caravan you're in.) Two additional rules: when one PC in your tree goes up a level, you can pick another PC of equal or lower level to level up at the same time; and (IIRC) everyone in a given player's tree has to have at least one dimension of alignment that they are. E.g. all Chaotic, or all Good. Different players don't have to share an alignment but one player's characters have to have some basic shared philosophy, IIRC.
Beyond that, it's up to you what if anything is shared by the PCs in your tree. They could be brothers, or people from the same village, or maybe they all used to be gladiators in the same stable, or they could have no more than a passing acquaintance one with another.
So that's what a character tree is.
The 5E DMG suggests that you make all PCs in the character tree level up when one does, but that seems cheap to me and results in everyone always being the same level all the time. I'd rather embrace heterogeneity. YMMV though.