The consequences of the 'accident' vary according to the nature of the incident and too many unknown factors which couldn't be answered here - You should consult with an attorney for a specific legal situation of this nature and, with your uncle for his reaction- those consequences must also be a burden right now.

Regarding the 'joyriding' -

Joyriding' is what lawyers call unlawful use of a means of transportation.  A typical example would be a kid who sneaks out of the house at night and drives a parent's car without the parent's knowledge. This is a class 5 felony. If you are a passenger in a car that you know is being used unlawfully, the offense is a class 6 felony. Both types of unlawful use carry a mandatory license suspension until the person who took the car turns 18 years of age. See A.R.S. § Section 13-1803 for the definition of the offenses, and A.R.S. § 28-3320 for the license suspension.