Do sifting and whisking do the exact same thing?
Sifting and whisking are not the same thing. Sifting breaks up lumps in dry powder ingredients, mixed powdered ingredients, and makes the flour less dense. Some people would say sifting makes flour "airier". I measured the volume of flour before and after sifting and I found a significant increase in volume for the same weight of flour post-sifting. Sift flour when making light sponges, or if your flour has gotten densely packed over time. 

Whisking is just a way of quickly mixing dry ingredients together, it doesn't break up lumps like sifting or mix things as effectively as sifting, but for many recipes it doesn't matter.