You probably know the gist of the history of A Nightmare on Elm Street. Freddy was a child murderer (which was kind of code for "child molester") but escaped jail time on a technicality, only to be burned alive by the parents living on Elm Street.
Ah, yes. Much like Beauty and the Beast, it's a tale as old as time, but there's a part of this story you might not know about from an earlier shooting script. Originally, in the scene in which Nancy's mother, Marge, recounts the vile history of Fred Krueger, she drops one other truth bomb that never made it into the final cut of the film. Fortunately, that scene did appear as an extra on a laserdisc release, so you can watch it right now before I give my super-important film critic opinion about it.
So, how do you like that? Nancy wasn't originally an only child, and her older sibling was killed by Krueger. Puts quite a different spin on the narrative, doesn't it? All of a sudden, this dreamlike horror ballet becomes more of a revenge tale. And I suspect that's exactly why that plot element wound up on the cutting-room floor. Yes, there is a certain amount of revenge in the final act once Nancy is the last girl standing, but even then most of the film feels like a dream. I'm not sure Nightmare needs to be grounded in reality with a dead sibling plotline.
Regardless, it is an interesting little piece of horror history. And maybe you'll love this moment so much you'll always think of the movie with this added moment in mind.