You can do the job with that camera. I recommend a few things.
First, you want an external -EX flash unit. Something like 580EX, 550EX, 420EX, or something completely compatible. The flash unit should be on a flash bracket, which is basically a handle to hold both the camera and the flash. You'll need the correct off-camera shoe cord to connect the camera to the flash.
Lenses? In general, you'll use a lens with an effective focal length of 50-75mm (figure in the 1.6 factor). For large family group shots, you'll need something wider than that, like 28mm. I had to shoot some really large groups at a wedding, and I had to go to 20mm for that, so you have to find out in advance how large the groups will be.
Once in a while, I will use a long lens for candid shots at the wedding reception. So, something more like 150-200mm works there.