While I feel it's probably not a good idea to shoot your sister's wedding, you've already agreed to shoot it. That being said, I agree with the above posts that you need to educate yourself as how you're going to light it. My suggestion would be to look around this forum and find tips on bouncing flashes, how to "chimp," how to drag your shutter. Learn about white balance. Technical stuff is very important in situations like these. Visit the venue where your sister is hosting her wedding, and see what you're dealing with.
As for gear? The wedding photographer in me would tell you to get a second body, wide angle lens, midrange zoom, and a midrange-long telephoto, in addition to a couple of flashes, backup batteries, and maybe 6-8 gig's worth of memory. Practice, Practice, Practice. Rent the stuff if you can beforehand and get a feel for how they work.
My family knows how I feel about them asking me to shoot a wedding. I RARELY do it. For one thing, they treat you like.. well, family. I would prefer to go there as a guest than a working photographer - and while I enjoy photography as a whole, I would also enjoy being a guest for a change.
Good luck! Shooting a wedding can be both fun and strenuous at the same time, but it's fun regardless.