Ok getting another photographer to help me like you guys are saying seems like a great idea and I'll most likely go in that direction.
That would have been my first suggestion as well. You just can't do this by yourself.
I've been to four weddings where I was very close to either the bride or the groom's family and thus would have been invited anyway. I brought ALL my gear and basically acted as an independent unpaid backup photographer that nobody really knew was going to produce any useful output (I was "the friend that will follow the other camera guys around" and that was about it in terms of introduction).
I can say that this is a really good place to be. At least one shot of mine from three of those weddings is sitting under glass on the couple's mantle, or in grandma's living room, or a similar prominent place in someone's house. It's really nice to see when you visit; makes you feel warm & fuzzy

. I find that your emotional connection with the wedding party makes it possible to express something with your shots that is rarely found outside close friends & family.
Now, the reason I was able to get those shots in the first place is that I talked to the hired pro well beforehand, explained who I was, got a feel for what she wanted to do, and then stayed the hell out of her way for the remainder of the event so she wasn't annoyed by my presence (& in one case even helped me out with the crowd once or twice).
My girlfriend was my assistant, and also ran around & took a lot of shots herself when I wasn't using her camera as a 2nd body. Her help was indispensable and often made the difference between getting the shot & not.
Generally, most of the other advice has been quite good. There are clearly some very experienced folks around here. I'm going to make a few suggestions based on my experience:
- it bears repeating: get with a buddy that shoots Nikon as you do, and do the event together. Discuss your roles well beforehand (he's your "assistant" and "backup"), establish that you are the lead photographer but that you value their help & couldn't do it without him/her.
- At all six weddings I've attended since I started paying attention to photography, either the bride's family and the groom's family both hired a photographer, or there was a videographer in addition to the main photographer (in one case it was both). Get your cousin to talk to the groom's family and make it clear that you can't be responsible for handling the photos for both families. You probably know everyone on the bride's side, but there will be a whole lot of people on the groom's side that you won't know and that don't know you. You have no idea if everyone hates his aunt Linda and doesn't want to be reminded that she was even there, and you don't want his mom telling you this kind of crap on the morning of the big day when you're trying to concentrate on more important stuff. Even if it's just you and the groom's videographer, he'll be the pro and at least some of the pressure will be on him to make things flow properly, and you can encourage & suggest as necessary.
- You need two camera bodies. The second one is not a "backup", it's a necessity. When it's go time, I've got my 40D with the telephoto on it for close-ups, and my girlfriend's 350D around my neck with the wide-angle for ambient and group shots. If you're switching from indoors to outdoors a lot you can also set them up so one is ready to go for each set of lighting conditions. The better of the two cameras gets the telephoto.
- If you aren't already using digital asset management software like Lightroom or Aperture, now's the time to get it & learn it.
- Really, don't take money for this gig (other than maybe gear rental).
- Try to get yourself invited to another wedding in exchange for some shots well before your cousin's wedding so you can get the hang of this (and maybe even your buddy if you can swing it).
Good luck
