After just reading the title I thought, "I doubt one youtuber can do something like that by themselves" but then you mentioned Pewdiepie and I completely forgot that insanely popular dude exists. His popularity is freaking insane, it's no surprise that he has that much effect on game sales. At this rate developers should be paying him a commission haha (although he probably doesn't even need that considering his reputation as the richest youtuber out there).
So, yes, it definitely happens. That's why Nintendo's idea of destroying that youtuber's channel who played hacked games was such a bad idea. Not only do these promote the consoles, but they bring in new audiences who had no interest in it before, like me. If not for certain youtubers I would never have gotten interested in Pokemon since it wasn't a significant part of my childhood like it was for most of the other current fans out there. Not only did I buy the 3DS and play the latest games, I also bought the older games which I played on emulators just to re-experience them again on the 3DS. Youtube is a huge place and very efficient at promoting games, whether through ads or sponsorship or reviews or whatever. It has an impact that can't be ignored.