Congrats on shooting the wedding! Here are a few of my ideas...
The light looks EXTREMELY low in some of those pics! I think you may also need to bring your tripod and/or monopod (monopod might be better for action shots, b/c it's more mobile and allows you to move quickly when necessary). You should also bring your best flash and some kind of softening device, like a Gary Fong Lightsphere (which I knocked in a previous post but still do use in certain wedding situations.)
For group shots at the altar, you can put the camera on tripod and/or monopod and get great, shake-free and blur-free shots at a lower ISO. If you absolutely need to handhold, you'll need to bump up your ISO quite a bit, I would expect. The D80 is a great camera, but it DOES show noise at the higher ISO's. So I'd def. try to use the tripod so you can get away with slow shutter speeds and low ISO when possible. The ceiling looks quite high so you won't be able to bounce your flash for most shots; that's why I suggested getting the diffuser. If you need to use flash pointed straight forward, that way you won't "blow out" people's faces if you're doing a nearby shot, and it will soften the shadows. The diffuser won't work well for "far away" shots though; it eats up too much light.
Since it's small & mostly family, I think you'll be fine! I'm sure they'll all be cooperative and follow your lead to take pictures. That's the good thing about family.
It's comfortable and they'll listen (hopefully!)
My other recommendation is to visit the chapel ahead of time if you can, and take sample pics at the same time of day that the wedding will take place. Maybe your bro will help out by being a model ahead of time!
That way you get to practice and see what kinds of setting you need to use, and which shots NEED the tripod to avoid the blur. That will help so much and will put your mind at ease on the day of the shoot.
If you can't visit that exact chapel, I'd recommend visiting a nearby church or chapel and just take sample alter pics & action shots in there. At least you'll get the practice of shooting in a dim, "churchy" environment and likely many of the settings you'll need will be quite similar.
good luck and have fun!
Jennifer
I agree with Sam that you should def. rent the lens; that way you'll have more options. You may also want to rent a 2nd camera body so you can have one lens on each camera and can swap quickly when needed. Like Sam said, there is often no time to change lenses.