I wouldn't fret it. Just make sure your neice understand what she's going to get. If you don't have your 580 by then and have time to practice with bouncing, etc., you'll obviously be using your flip-up flash which is fine, but will get you that limited, harsh lighting that comes with small on-camera flash. For pictures taken as a favor by an aunt, she should appreciate having them regardless. However, you might want to show her some samples of your flash photos in similar situations, maybe in a dark restaurant or something, just so she knows what they'll look like.
As far as what to shoot, I've never done a rehearsal dinner myself, so I'm not speaking from experience. I'm surprised she asked someone who's in the party because it might be nice to have shots of the actual rehearsal. Maybe there's someone you can pass your camera to for a little while? Then I think you mostly take candid shots of the dinner as it happens. Maybe a couple of groups shots not unlike a wedding: the couple, the couple with each set of parents, and then the full wedding party. I would keep it simple and not stress out over it.