I think it's possible that it was a car-jacking gone wrong - hold them up, gun goes off possibly by accident, kills the driver - car-jacker panics, shoots all the visible witnesses, cyclist appears on the scene, car-jacker has to shoot him too then flees the scene.
It's not unbelievable.
It woudl be very hard to believe that it was a family member if they were all on holiday; but if they lived in the area, then more possible.
I just hope the older sister pulls through as well so that they at least have each other.