I have this old 3ds that my little brother treated very badly. Can I do something to fix that?

 

Ok, but I don't know if the problem is the top screen, the bottom screen or maybe a cable.

 

It's probably the bottom screen or the bottom screen ribbon cable, I think it's more likely that the backlight would go darker as a result of damage than brighter.
But, what brightness level was the 3DS on in that picture? The bottom screen looks like it's on brightness level 1, and the top screen looks like 3 or 4. If it was on the lowest brightness level then it might be the top screen, if it was on 3+ it's probably the bottom screen.
I can't be 100% certain without seeing it in person though, since camera exposure and brightness and screen brightness will make the image look different on my screen than it does in real life, at least to some degree.

 

Yeah, that's definitely the top screen.
You most likely only need to replace the ribbon cable, which is a cheap fix.
But maybe the chip that regulates the screen brightness for the top screen is broken, which would require the entire top screen assembly replaced.
Anyway, I suggest reseating the top screen ribbon cable like someone else here already suggested, and if that doesn't help, try replacing the ribbon cable entirely. I've heard it's a pain to replace, because of how it twists through the hinge, and you have to be really careful not to damage any of the other ribbon cables while disassembling or reassembling the console. It's worth a try though, since the ribbon cables are so cheap. If you can find a pack with several of them for not much more than what 1 costs on its own, I suggest buying multiple, as they are a pain to replace like I said.