I blame [MENTION=607]Klaus[/MENTION] . In a
Dungeon adventure cited as inspiration for the adventure, he put a group of dusk elves in the forests near Vallaki. Kasimir and his sister are basically ripped right from that adventure, so he'd probably be the person to ask.
The elves themselves were introduced in 4e in a Dragon article by Robert Schwalb, and in the presumed setting there they were eladrin who refused to take Corellon's side in the war against the drow and, when his anger turned on them for that, they were protected by Sehanine (who, I guess, wasn't as angry at the Drow as Corellon was?). They're now all emotionally dead and suspicious and kind of emo about not being eladrin anymore.
In the fiction, I suppose Barovia was one of the places they fled to in the war against the Drow, and then they just hung out there until Strahd came along.
Mechanically, they were an attempt to do something a little experimental in 4e - make subraces based on feats (4e did Vistani the same way, actually...). As a dusk elf you were exactly the same as any normal 4e elf (wood elf), except you spent one of your feats on something that gave everyone +1 to Stealth within 30 ft. And then you could spend some more feats to basically reward you for finding concealment and being invisible. And there was a paragon path mechanically about being invisible / concealed and fictionally I guess about being some sort of elf vengeance ninja.
In 4e, for most of the time you played, you had better things to spend your feats on than a racial feat that didn't benefit you much, so I didn't see them played a lot.
But
Curse of Strahd has certainly re-invigorated interest in them.
In 5e, they'd probably just be wood elves. If you're being very technical, maybe replace
Mask of the Wild and
Fleet of FooT with something like
Dusk Elf Stealth. You gain proficiency in Stealth. Allies within 30 feet of you can add half of your proficiency bonus to Dexterity (Stealth) rolls they make that don't already include their proficiency bonus.
Edit: Fleet of Food?...like some sort of laxative?