This is the classical description of friction. As you know friction acts in the opposite direction to the motion and is measured parallel to the surfaces that are touching. Friction is proportional to the load or the normal force 2. Depends on the nature of the surfaces 3. Does not depend on the apparent area of contact of the surfaces 4.
Friction is much more complicated than the classical description suggests.
Scientists have been trying to figure what actually causes friction for centuries. In fact the first experiments on the subject were conducted by di Vinci. In your sisters experiment she found that as she increased the roughness the friction increased to a point then friction started to decrease with increasing roughness. It has been experimentally shown that the friction may decrease with roughness and in the other extreme very smooth surfaces may have enourmous frictional forces, this is what happens in cold welding. This seems contrary to the classical explaination because it has been shown experimentally that friction depends on the contact area of the two objects.