My opinion may not have much credibility, cause I am not sure where the rockgarden really is(or Provo canyon to keep this within the thread geographically); I wheel a minvan(an IFS minivan at that-but its a Toyota at least); and currently I drive a Ford.
Nuff said, yeah, bad choice to give him the keys. Good choice to stay sober, which led to clear thinking, which led to the judgement call of the circumstantial presentation, (doggone I hope that came out right, vocabulary is sooo not what it used to be) of your decision to let him access your jeep.
You did not give him express permission to roll (no pun intended) to the Maverick for Marlboro's.
You thought you could trust him, after all he is your "brothers" (quotes for a reason, I have followed the story/lineage of participants) cousin, and that should account for something. To use a first person analogy, had it been me there that night, I never would have gotten the keys from you\you to give me the keys, as you don't know me from Adam. Hence no implied trust, as there was with "Bob"
In the same situation, I would have done the same thing, and I am 15 years your senior, not that means anything, but it is said that when we are younger, we do things that we would not do with a little more real world experience. You thought there was trust, you had faith in the out come of your decision, and you have taken accountability for the initial wrong doing, giving a drunk man (kid?) your keys.
Sorry for your loss.
Hope you get to wheeling again soon, and good luck on the parting of the
j(h?)eep