My mom is a wedding photographer and from 13 to 15 she brought me along as her assistant until I could get a real job. So I've been to around 50 weddings as a helper.
1) It's your cousin, so you can totally socialize, but realize that you are getting paid, don't forget that. If you're in a conversation and you hear someone getting ready for a toast, you should also be ready.
2) Talk to the other photographers and figure out where each one will be during the procession. There needs to be a photographer in back and in front. You'll likely want whoever is better in the front, so likely you'll be in the back if there is someone who has shot weddings. So you'll want to be getting the candids right before. Usually a nervous photo with the dad and so forth. Then try to get a back shot of each pair.
3) Realize that you can move around. You need to be quite, but they'd rather have the shot and you moving around a tad than you being totally silent in the back trying to zoom in for the shot.
4) Artsy isn't better, don't try to take "artistic photos" you can add stuff in editing if you really must. Your ruining what may have been a beautiful shot by including a flower petal in the frame. Unless the photographers are off doing wedding party photos... then take photos of whatever you can think of, more is better.
5) Take more photos of the bride than you could ever think of. She's the one who wants to look at them, as well as the mother of the bride. So if you can get some nice shots of both of them, they will likely be stoked.
6) Pay attention to what friends really seem important to the bride or groom. Usually they can't have a wedding party as large as they would like, so friends are assigned to other tasks. Make sure you get photos of the bride/groom with these people. They are important as well. You can usually find them in the program as greater or attendant or about 20 other titles. You can also ask the Maid of Honor, it's her job to know these things.
7) Try to keep questions away from the bride and groom. They are already stressed, ask the bridesmaids and groomsmen, or wedding planner if there is one. They are the preferred contacts at the wedding. BEFORE the wedding, you should ask if there are any specific photos the bride/mother of bride want. Also ask the groom. Sometimes there is something special being planned that 1 party doesnt know about.
8) If you are doing any portraits feel free to have fun with them, but always pay attention to things like hands/elbows, what's in the background, that there isn't lipstick on the teeth and so forth. Sure you can edit these things out later, but it's better to not have to edit 100 photos with lipstick smudges. Also pay attention to the brides train if she has one. You usually want to pull it infront and spread it out so it looks all pretty.
You can get a list of crucial wedding shots online. No one is going to know that you have it or that you are using it as a guide. Bring it with you, make sure you get them all.