Beautiful capture. Love the bright colors - everything pops!

Hi there! I'm new to the site but have been perusing posts and wanted to send some encouragement your way. I'm not a professional photographer by any means - more of an extreme hobbyist - but have been asked to shoot events, including a few weddings over the years. I always take photos at the reunions and family events but only because I enjoy it; and, I feel like this experience helped me when doing the weddings. Some of the very best shots I capture are candid shots and they always seems to really lend a feel for the mood. People are relaxed and genuine and that comes through in the photos.
You mentioned that this is a casual affair so I would imagine that a lot of your shots will be similar to taking photos at family gatherings. I would suggest getting a shot list from the bride and groom so you know what their "must-have" shots are, and also make sure you have a solid grasp on the schedule so you will be ready to get the absolute must-haves that they want. It wouldn't hurt to have your niece send you a couple of wedding shots that she & her beau-to-be really like (there are thousands upon thousands online via google search or check out her pinterest if she has one!) so you can see what style really catches their eye, too.
After getting all of their must-have shots, you can just drift through the party shooting great moments between fam/friends - there will be a lot of hugs, a lot of laughter, and a lot of happy to capture!
Attending their rehearsal will be super helpful so you can see where they will stand, which direction they will face, etc. Plus, it will be helpful for you to see the house/property where they are getting married beforehand, even if it's the day before, so that you can make a note of any spots that you see that will be great for photos. Be sure to look around and take test shots at the same time that they'll be having the ceremony so you can be ready to deal with the harsh lighting & shadows the day of. Then you'll have a better idea settings-wise of what to expect and can spend more time looking for the photo opportunities vs. the back of the camera.
I would bring both lenses, the flash just in case, & definitely a tripod. If it's tough to travel w/a tripod then maybe another guest wouldn't mind bringing one for you to borrow. As always, bring extra batteries for your flash (I was surprise with how fast my flash burned through batteries at one wedding and was left scrambling to get more), extra camera batteries, and extra memory cards. Oh! A circular polarizer is a must-have of mine and I never shoot anything without one. It will reduce glare and help make the colors pop.
The best part is that if your niece is already a fan of your work then she already likes your style and the photos that you take. They're going to be thrilled to have you be a part of their day.
Apologies for a long (first!) post but I hope that this was helpful and put you a little more at ease and confident that you are in fact the perfect photographer for them.