Adding an empty gameobject to my rigged hands break the movement

Hello guys! I am testing random things in a Unity 3d project. I'm currently trying to equip an object in my player's hands. I made them in Blender and I rigged them also in blender. I created the animations with the Unity animator, and everything works fine when my hands has nothing attached to them. However, I'm now trying to equip an item to my right hand, so I'm creating an empty game object on my right hand as an item placeholder (I will instantiate an object via script in runtime). But when I load a basic 3d object as a child of the placeholder, the whole arm stop working correctly. Every pivot of the arm (the center point where rotation and movement of my arm, forearm, and the hand itself is) become the same as the hand item placeholder pivot. If I try to rotate the hand or the forearm, the whole texture breaks and joints stop working. What can I do?
It's pretty hard explaining what goes wrong (expecially to a non-english speaker), so I've made a video showing what's wrong: https://www.youtube.com/watch?v=nEeLeTlV5rc&feature=youtu.be