If a driver taps the brakes when there is no road-relevant reason to do so, I suppose it might or might not cause an accident. It might even if it was a light tap, because brake-tapping is used as a communication device with other drivers, who might misinterpret your intentions and act accordingly and set off a chain reaction.
This was exactly what I was trying to say, and thank you for putting it so clearly! The drivers around you have no idea what's going on inside your car, all they know is that your brake lights flashed and they don't understand why and don't understand what you're trying to signal to them. It's a whole different matter if you're tapping your brakes in order to signal to the other drivers, or just because the driving conditions required you to step on the brakes for a second. I also don't think it has to be a violent tap in order to jostle the passengers or wake up a dozing person. Maybe it depends on the specific car and how jerky the brakes are?
I do think there are people who have the ability to nudge someone in the back seat without being distracted, taking their eyes off the road, or fumbling around. My mother was excellent at that, during my childhood! But if the sleeping is distracting, and he can't take care of it without confusing the other drivers or distracting himself, then I think it's time to pull over and settle this when the vehicle is stationary.
I agree that the first time OP chose to ride with her friend, there was some bad communication, he didn't think anyone would sleep in the car and she didn't realize it would bother him at all. But after that, I think it's a different situation than accepting the ride knowing that the sleeping is a problem, and then making plans in advance to manipulate him into getting things your way. Not everyone, even the closest of friends are going to be compatible for every activity and I don't think it makes them bad people, just bad road trip buddies.
Also, it depends on the aunt! Aunt June who is sweet as can be, absolutely not. Aunt Heloise who has been venting for hours about offensive fringe political and religious views, I might be tempted.