So what have I to tell to my brother not to touch ?
Probably the best thing to tell him is to not uninstall any of the Games you've installed, since it'll be a pain to reinstall.
possible to hide icon of apps as fbi or luma updater ?
There is a semi-complicated way to hide these Applications, but it is doable for a normal user like myself so it should also be manageable for you.
The solution I'm suggesting is, before installing any Games, to install
3DSBank and to then create a new 3DS Menu where you'll install all the Games into.
The 3DSBank Icon is pretty inconspicuous so you can place it into a normal Folder and nobody will think it's out of place.
What a new 3DS Menu does is it gives you a new, blank slate, where only the default 3DS Applications show up and others like Luma Updater and Homebrew don't; to get into Homebrew, you'll have to assign it to the Nintendo eShop Application, via Luma.
Additionally, once you have 3DSBank, you can overcome the 300 Title limit by creating new 3DS Menus and installing more Games, as long as your Micro SD has enough space.
I hope this helps.