I shoot weddings all the time, and--like Southern Cross--I don't know how to give you technical advice without know what you're working with, but much of this will come primarily by feel. I know that doesn't make you feel at ease. First of all, have you been to other weddings so that you're familiar with the usual way it works out?
I usually arrive wherever the bride is 90 minutes before the ceremony so that I can get any "getting ready" shots and perhaps save time by getting the pictures of the bride and her bridesmaids out of the way before the ceremony. Since you're a member of the family, you may be there anyway.
Then there's the ceremony and you can guess the obvious things to remember: the procession to the altar, the first kiss, the exchange of rings, the presentation of the married couple, throwing of rice/bubbles/people (jk), etc.
Afterwards is the usual time for the formal pictures of the bride with bridesmaids, groom with ushers, bride and groom with entire wedding party, bride with her family, same with groom, bride and groom alone. It is best to plan this out with the bride and groom before the wedding rather than trying to organize it there.
Then there's the announcement of the bride and groom at the reception, the toasts and usually people get down to the eating. First dance, garter and bouquet toss, and the cake cutting are really all you have to worry about, but most couples like shots of the guests enjoying themselves and you definitely have the advantage of knowing some of the guests, so that will help.
I find myself shooting 8-10 rolls of 36 exp. on a normal shoot. That may seem like a lot, but it goes by quickly. Keep in mind that during the ceremony, you want to keep an eye on the pictures left on the roll. If you've only got 7 pictures left and they're getting close to the rings/kiss/announcement/etc., then you want to just sacrifice those 7 pictures and just rewind and load a new roll. It's better to sacrifice the frames then miss the first kiss because you're in the middle of changing rolls!
I'm sure I'll remember more tips later. Just make sure you have fresh batteries in everything. Turn your cell phone off! Wear very comfortable shoes and an outfit that will allow you to stand on chairs if need be or crouch low to the ground. Comfort must reign in this profession.
Go to the library or Barnes & Noble and look at wedding magazines and wedding photography books. You won't remember 98% of any posing tips that you read about, nor will you achieve the perfection in wedding magazines which have been posed, professionally lit and airbrushed. However, the most important thing is to look for moments and small details which really tell a story about the day. The bride's shoes, the bow on the back of the flower girl's dress, the flowers at the reception, the wedding favors, etc. Everything is important...because that couple thought it was so special they paid for it!
I just started an online site for my pictures, which you can go to to see one of my weddings a couple years ago. Visit. w(3).pbase.com/photojournie
Whew! I hope that helps! Email me if you need more info or have more questions.