Sheri, it sounds to me like the over anger and aggressiveness is a defense against the anxiety and "helplessness" of your aunt's situation.

I use to have rescue fantasies where I was in desperate straits and pulled through and I figured out from them that when I'm feeling cornered or helpless, thoroughly depressed and defeated (I think of it as being "painted into a corner") I get angry. I'm not normally an angry person so when I get road rage going and notice it, I know to immediately stop and think what is making me anxious, where I'm feeling helpless. The anger "motivates" me and keeps me going until I can get "help".

I wonder if talking about and remembering your cousin in positive ways would ease some of your aunt's "obsession" with the driver and accident and her guilt. If you can, "wishing for" your cousin to be there. . . because of her sense of humor, or helpfulness, or kindness or some other positive attribute of hers that would be good to have in a certain situation, might help your aunt "reclaim" that characteristic. When we love another, we invest ourselves in them and death is death also to our investment of ourselves which is part of why mourning is so painful and difficult. If you could "model" good mourning perhaps she could learn from you or "use"you as a temporary holder of her investment in her daughter, etc.

Anyway, that's what I would try; anytime the accident or driver, trucks, etc. gets mentioned I'd try to change the "subject" so it is more on the positive attributes of the daughter and "sweet" remembering/mourning.