I want to make it very clear before I say what I'm going to say: I MEAN THIS IN THE MOST POSITIVE WAY IMAGINABLE!!!
This feels like something I would have made when I was a kid.
Maybe not a KID-kid, but like, mid-teens. I'm not that far off from that age (I'm just in my 20s!), but it reminds me of all of the best parts of my projects from that age. A story that's more small in scope, but very character-focused, and emotionally resonant, for that reason. (You even had an emotionally stunted blonde! That's, like, my favorite type of character, ha!) It feels like something that was made... how to say... unabashedly? I'm worried that this is coming off wrong, but I mean this all so, so positively!
I think what I'm trying to say is, it feels like something I would've made as a kid in that it knows exactly what it is and what it means to do, and it does it well. I think a lot of other stories like this pad it all out with a bunch of unnecessary filler, because that's what people think makes a story "good", is when a story is very... thorough, I suppose. (Which is a pattern of thinking I've often applied to my stories, which doesn't always help.)
But we didn't need to know the exact details of the magic system, and the exact details of Cain and Sammy's backstories. We got glimpses of what we needed for the story to be effective and resonant, and our imaginations fill out the rest. That's often the best way to write a story like this. The character writing is very strong, and I just generally had a darned good time with it!