I wouldn't rely on RAW or HDR. Try to get the lighting as good as possible at the scene. Have a plan. Be ready to adjust your plan because reality rarely cooperates.
1) Make sure that the sun is going to be where you want it to be. Check a sunrise/sunset azimuth calculator to see exactly where it will set. For general purposes in the US, it will set somewhere in the west or southwest.
2) If the sun is behind her, she's going to be a silhouette. Unless she is made of stone, HDR isn't your answer. Use your flash for fill light. The trick is learning to balance the light from your camera and from your flash. If you leave it to the camera, it'll nuke her and let the rest of the scene go dark. You need to either know how to shoot your camera in manual and your flash in manual or know how to dial in exposure compensation and flash exposure compenstation. Personally, I'd go with full manual and practice it enough to be able to make changes quickly as conditions warrant.
3) Assuming that you use your flash to fill, the light coming from it will be much cooler than the light from the sunset. It will look odd to have her light with white light during an orange sunset. It
might look interesting from an artsy perspective to have the different colored lights, but it definitely won't look natural. You want to put an orange gel over your flash. Get a CTO, 1/2 CTO, and 1/4 CTO. Bring them all and try different ones to see what works the best. They are cheap and easy to find.
4) If you can, get the flash off of the camera, direct fill is better than nothing, but it isn't very interesting. Neither the Rebel XS nor the 430EX have sync ports, so the easy option of just buying a long sync cable isn't there. If you've got time, you could order
one of these. You could also rent a 580EX or an ST-E2 to use as a commander. The interface is pretty confusing, so if you go the rental route, give yourself a day or two for practice.
5) If you can get the flash away from the camera, you also want to diffuse it. A blast of light from a small flash isn't very flattering. If you can get something translucent and white to shoot through. I like to use a small, shoot-thru embrella, but anything that will make your flash bigger will help.
6) If you get the flash away from th camera and diffuse it, you will also want to control it. Having gobos (stuff that goes between your flash and your subject to block light) helps. If can be as simple as a little gaffer tape on the flash, a piece of cardboard, of whatever you can get your hands on. As a general rule, you'll want the flash at its most intense on your subjects face and have it very slowly fade away from there.
Here's a vaguely similar shot I took last year. OK, it doesn't have a bride or a pier, but it does have a sunset and it illustrates some of what I was talking about. It was taken in manual mode on the camera. I don't recall if the flash was in manual or E-TTL. I'm guessing E-TTL at -2/3 FEC, but that's just a wild guess. It was fired through a CTO or (1/2 CTO) gel and a shoot-thru umbrella. The flash was set as a wireless slave on the end of a paint pole held by my wife acting as a VAL (voice activated lightstand).
Sunsets aren't predictable. Sometimes they are pretty. Sometimes not. If it gets really cloudy, what are you going to do? If there are no clouds at all and the sunset has no drama, what will you do?
I took this shot of my niece a few weeks ago. We had a dull sunset. The sun was way behind the clouds long before it set, so the light was nothing special. Using a gel helped give your a warm light. Selectively lighting her also helped. The concept was the same as the prior shot, but nature wasn't as helpful.
To summarize: Make sure the sun is going to be where you want it. Moving the sun or a pier at the last minute isn't practical. Use your flash for fill or you will have a silhouette. Learn to control your flash so that you can balance the ambient light and the light from the flash. Gel your flash to get the color of light that you want. If possible, move your flash off camera and diffuse it. This isn't as critical as the stuff before, but it will help. If you can't get there this time, start planning on how you'll get there for next time. Finally, learn to control your flash(es) so that light goes where you want it and not where you don't want it. This is last part is one thing that you can fudge fairly easily in post production.
Finally, be ready to make it up as you go. After having watched bunches of videos from incredibly talented shooters, I'm shocked by how often they make radical changes to their plans. When you are making it up as you go, it really helps to know how your gear works.
One thing I will pass along, when you use a longer lens for portraits, I zoom in on the eyes and make sure that they are in focus, then pull back out for the shot I want. 
