So a player in my game was playing a character heavily based on Number 5, and we as part of it needed a bunch of NPCs that were the rest of the siblings.
His backstory was that he was from Waterdeep and was one of the minor noble families that was into tinkering and making mechanical objects. The player used a homebrew class for 5 called the blink mage (we found it off DandDwiki.com and then made it actually balanced), whose core mechanic was gaining the ability to cast misty step endlessly and had a mechanic called "blink charges" they could use to augment said jumps. It was a bit OP for the first few levels, admittedly, but after level 4 or 5 it was fine so I didn't care.
Basically his backstory was he jumped forward through time into an apocalyptic future where Waterdeep was destroyed and he knew it had something to due with the Blood War (I'm running dragon heist and eventually going into a Descent into Avernus game). He ended up having a similar handler to the woman in the show/comic that I used Shemeshka the Marauder from Sigil who basically taught him how to kill/investigate (and loved to screw with him because...it's Shemeshka) and promptly used powerful magic to send him back in time when the game is set to keep the world from ending. The organization after him is associated with another faction from Sigil, the Modrons, which don't want him messing with the timestream. Ain't perfect planescape cannon, but who cares.
The siblings I made as such:
Luther uses the stats of a Bargula Demon except he's technically a humanoid. I made his injury and Ben's death have occured at the same time (I made several changed to simplify things because the other players weren't playing the siblings) as a mission gone wrong after Five disappeared. Basically the group fought a Bargula demon and it killed Ben and crushed in Luther's chest. The games version of Reginald was an artificer that was secretly from Ebberon and used the demon's heart to keep him alive and it...had side effects. He was locked in a prison after killing a servant of a noble and assaulting another to simulate him basically being isolated for multiple years.
For Allison I made her a succubus and gave her bard spells focused on charming and mind control. It's the best way to simulate her rumors in game. I made her an actress in one of Waterdeeps theatres.
Diego took a bit more home brewery. I basically made him an Air Genasi and switched out the racials for casting cloud of daggers 1/day and made him sort of a rogue with abilities from arcane archer (seeking arrow, grasping arrow, etc) and just ruled he does it with throwing daggers. It works quite well. I tied him into Dragon Heist as living in the inn ran by another player's parents and working as a vigilante by night (what they were originally doing as a group) keeping the streets of Waterdeep safe. His latest annoyance/potential love interest? The Black Viper, a jewel thief that kept eluding him in a sort of Batman/Catwoman esque way.
Klaus I made a Drow (for levitate as a racial and because it fits that 'shady' vibe of him) undying warlock with pact of the chain and made Ben just a ghost (that could cast Hunger of Hadar 1/day) that was effectively Klaus's familiar and wouldn't be permanently exercised unless Klaus died. I decided to replace Gnarl from Xanathar's lair with Klaus because he'd have to get his drugs somewhere and it gives a fun element of complication for the party.
And for Vanya I just made her an Eladrin Bard and gave her an aoe psychic attack (from Rahadin from Strahd) she can do by playing the violin. I kept the angle of her having her memories messed with because she was emotionally unstable as an Eladrin that would basically change seasons (and this have WILD moodswings) everytime she used her teleport ability. She wasn't related to the apocalypse in any way for this game, merely just some way of preserving the feel of her potentially going nuts. With Vanya I sort of changed the most in that she was sort of the groups healer until she went nuts and everyone went their separate ways.
Probably not exactly how I'd roll them all as PCs, but enjoy.