The media report linked by Pace adds nothing. These were air experience flights and it would not be unusual for the cadets to be given control. All cadets from the same squadron "know" each other so the fact that they were cousins would not in my view have made formation flight any more likely.
In any event, the aircraft were TXP equipped so I suspect their relative tracks and altitudes will be readily available to the investigators from the Cardiff Radar tapes.
As a CI for a fellow ATC squadron in the Welsh Wing, and having met both instructors involved in earlier air experience flights I've taken cadets to, this accident is very close to home for me.