On the water would be better, you can get instant feedback that way. My ankle is just about healed so I'll check back with you in a few days. Would you be up for that?
Steve is right, that will spin you backside, not frontside. Which is fine, but it depends on what rotation you're trying to learn.
I would say your brother has his rear arm a little too high on his back so he's not able to lean back and use his whole body to create load on the line. You see how his shoulders are rolled forward? If he was able to bring the arm that's holding the main handle a little lower, he will be able to square up his shoulders more and have a better axis as he leaves the wake.