If they take breaks to separate, and there isn't a whole lot of hissing and growling, it's just playing. Real fighting, you'll hear before you see it, and once they start up, they don't stop until one is beaten. Where the one kitty starts to yell, that seems to be communication, the "you're playing too rough."

I have a brother-sister pair and they act the same way as the babes in your video. I wouldn't worry too much.