Actually I build everything on my own (except models, meshes, stuff like this. god save my artist :D).
I am a game designer and game programmer. I do stuff normally on my own. I.e. if you want a level system, your brother can make a new Character class, with some new properties. I.e. float experience. Then add a uproperty int32 for the actual level. Then add a function maybe called addExperience(float exp). In this function you add the exp to your experience. And if your exp is higher than i.e. 100, you add a level, and reset the experience to 0.
The normal approach for developing is to take a big problem like “Inventory-System” and seperate it in many many small problems. Like items. Items u seperate in stuff like name, id, attack damage, defense… If you have created the code (in blueprint or c++) then you can think about the next step. maybe the way to collect it. After this you can think about where to store it (inventory). Then you have to think how you interact with the inventory.
Stuff like this. Hope it helps a little, even without any great link to a source 