Also I would have liked a better resolution. Why did the sister ignore the warnings?
I read it as, "She wants more than anything to find an example of true randomness because she feels weighed down by the way everything is predictable. She tries to escape the grip of her wyrd, and the only way to do that is through her death, in grand Norse tradition."
She goes to the accident because it is random and senseless, in other words. The terrible discovery her brother makes is that she wanted it to end that way, that she was in pain and suicidal.
I think that's a bit of an overstatement of what's in the story. First, as I still maintain, we have no evidence that the sister used SHELDON to predict her death. All we know is what it would have told her, had she done so. And we also know what it has to tell her now that she is dead. Which is revealing enough.
Second, even if she had read it, the warnings were quite vague - "a truly random occurance" does not necessarily mean "something fatal". It means "something special". She would have known that randomness brings risk, but by its very nature, the promise of randomness does not guaruntee any outcome, including death.
Third, the fact that when asked for good news for a person who is already dead, SHELDON replies with "good news, it's over", doesn't mean that that person is suicidal (though I accept that the narrator did interpret it that way). I am in no way suicidal, and there is a lot I really love about my life. But there is also a lot in my life I would be very happy to leave behind. SHELDON was programmed to give good news, and presumably "it's over" is the best news it could come up with. That doesn't mean it's what the sister would have chosen or wanted.