A few tips from a long time wedding photographer who has trained quite a few assistants:
Keep it simple: use gear you are familiar with, do not introduce unfamiliar gear, borrowed, rented or bought. If you do not have a back-up camera and want one, rent the same camera that you already have. If not, do you have a family compact camera? Take it also. The 70-200 is a staple for many wedding shooters ... it will help you get shots of everyone at play especially in the water. The 70 to 110 focal lengths of that lens will work for 2 person posed shots and closer portraits. a wide zoom like the 24-105 will do for all the rest. In fact, you could shoot the entire wedding with the 24-105!
Prep, prep, prep! Put together your kit the day prior, and test it. Charge all batteries, reformat all cards, test your flash, pack for action (put the 24-105 on the camera so it is ready to go immediately).
Practice, practice, practice. Is your niece near you? If so, shoot an engagement session with them which gets them used to you, and you can determine how to best photograph them. If not, recruit another "similar" couple to practice on. Keep in mind what body type your subjects are, and only study other posed shots with a similar body type.
What to practice: shoot a couple walking at you like a processional. DO NOT set your camera on continuous focus unless you are expert at it. I've found that these type of wedding shots unnerve most beginners because the original expressions cannot be re-staged later, and practice helps with feeling easier about them. Practice timing shots like the ring exchange and first kiss. These shots also happen once in real time, however if you miss them, these shots can be re-staged ... a couple in love will take any excuse to kiss : -)
Scout, scout, scout: Try to arrive earlier and become familiar with the location and more importantly how the wedding ceremony is situated in relation to the sun, or what the reception area looks like.
General tips:
Do not shoot larger group shots @ 24mm ... it distorts people at each end and makes them look fatter ... try to stay with-in 35mm focal length or longer if possible.
Where possible, seek shade ... shade without dappling or hot spots ... if in a wooded area use your flash to keep the green ambient shadows off the skin.
Key Tip:
Practice with your speed-light. Think of it as a supplement to the ambient outdoors and indoors, not as a main light. Outdoors it will fill shadows caused by downward sun that causes "Raccoon Eyes" ... indoors in lower light, set your camera to manual exposure and a higher ISO like ISO 800 or 1,000, and flash to TTL. As a basic starting point, set the shutter speed to the focal length of the lens : 50mm = 1/50th. This is called "Dragging The Shutter" and it allows the camera to record the lower indoor ambient while the speed-light freezes the foreground subject action because the flash duration is so fast (usually 1/1000 or faster). This method avoids the black background with over-exposed foreground subject so prevalent with beginner wedding shooters. Trust me on this, and give it a try to become comfortable with the technique.
Lastly, if or when you experience "brain freeze" or you become over-whelmed by some situation that you can't seem to solve immediately ... set the camera to "Program" and keep shooting. I tell my students that "P" on the camera stands for "Panic".
Best of luck ... and LUCK favors the prepared!
Marc Williams
FOTOGRAFZ, LLC
https://fotografz.smugmug.com/Fotografz-Weddings-Events/Wedding-Samples/Wedding-Samples/