A knife edge would normally be described as straight line flight with an angle of bank of around 90 degrees. As pointed out, if you wish to maintain level flight you have to do something to replace the lift vector now not opposing gravity - so you end up with the fuselage having to have its angle of attack (with respect to straight and level flight) increased - by yawing usually.
What your nephew is doing is a banked turn with an extreme angle of bank (certainly compared to a lot of GA). It is usually not a balanced turn as GA teaches it, and it is necessary to pull positive G (considerable amounts) in order to maintain height as well as change heading.
Fast Jets are required to have considerable capacity for rate of change of angle about all of their axes to do their work (for example evading missiles or Radar Lock or flying at low level). This is best achieved by rotating the aircraft angle of lift towards the direction you wish to go with t/ailerons, and then pitching to change heading.
A lot of the time rudders don't feature in a lot of fast jet flight (or in a lot of aircraft). When I flew Bulldogs the pilots would tell me off for trying to coordinate turns after having it (rightly) drilled in to me when flying Vigilants! For the most part there are much lower structural limits on vertical tailplanes than on wings. Partly this is due to a lack of requirement for Yawing at high speeds - I think the Harrier has a Yaw limit of 7 or 11 degrees for example above certain speeds. Someone here may remember. I didn't work on Harrier - I didn't want to live around Farnborough. Therefore you can't generate the turning moment about the aircraft CofG to change heading fast enough even if you wanted to, and obviously it doesn't make sense to 'skid' around a turn at any speed using yaw only as its very inefficient and potentially dangerous.
The downside of high G banked rolls is that you lose energy very quickly, and this has to be replaced during the turn using throttle to balance the massively increased drag (multiple sources) and to also accelerate away after exiting the turn. Depending on the loading state of the aircraft the G you can pull can be severely limited too, thus modifying what you can do with the aircraft.
You can explore these situations in RC aircraft.
A simple 3-channel powered aircraft will usually have a throttle, a rudder and elevator channels. This type of RC aircraft changes direction using rudder and aircraft configuration (usually high wings with a marked amount of dihedral) to take advantage of the secondary effects of control of yaw to induce bank. This is further controlled using pitch to assist with maintaining level flight.
A standard 4-channel aircraft will use throttle, rudder, elevator and aileron for control and can perform efficient balanced turns. If configured correctly it can also perform highly banked turns with aileron, assisted with pitch.
You can also operate an aircraft on 3-channel controls using throttle, elevator and aileron. These are more commonly known as 'Bank and Yank' aircraft after what you need to do to turn them.
This is a massively complex topic that I'm butchering but that is the dirty basics.