Ignore the naysayers and just go do it. You are going to make mistakes and have some nerves, but no one is going to die if you miss a shot. It's a second marriage and anything you shoot will be better than cell phones. Besides, anybody married more than 5 years hasn't looked at their wedding book in 4 and probably can't recall where the book is stored. Better a family member who will forgive you than a paying customer who will always blame you for the littlest mistake, if not want to sue you.

My dad moonlighted as a wedding photog when I was a teen. Was quite popular and had a wedding almost every weekend in season.
What I learned assisting him... Have a backup camera, and flash. Extra batteries. and most critical, leave nothing to memory. Write it all down. Shot list, shot list, shot list. and gear check list. Extra SD cards and check that you have one loaded. We had to re-stage several shots at his first wedding ever (which was a family member) because first rack of film didn't thread properly. He survived and went on to be the family go to as well as shoot several referrals professionally. He only worked by word of mouth.

It wouldn't hurt to round up a couple of friends and practice in your backyard with flash outdoors. Also, you might be surprised at the distracting backgrounds you see. Will make you more ready when posing for the real deal. Worst part for you will be that you are working and not getting to enjoy the event of a favorite niece. You will be thinking ahead and positioning for next shot the whole day and almost as tired as the bride at days end.

Good luck and try to enjoy the experience!

I'd go with a "snoot" on the flash to spread the light on fill flash. It limits the range so a large group might require direct flash. For TTL flash work, start with Manual mode, ISO 400, f4, and 1/200. Set your camera to allow high speed sync so,you can go to high shutter speeds to darken the ambient light. This is a great technique indoors or out. The TTL flash will control exposure on the subject as you change the other settings. A higher ISO can help extend the range of the flash and of course a higher f stop for increased DoF. I like to zoom in for candids in groups like this. Don't have to worry much about camera shake.

Since your friend has expressed confidence in your work and style, you are ready for a wedding. If you were to buy a second lens the 24-105, you would be good. If you are renting would go for the 24-70/2.8. Since it is informal, your Mk IV is enough. Would not worry about a back up DSLR unless you have one already, but it is always a good idea. You can always throw a P&S in your bag just in case. Flash? one on camera flash will work in most cases. Just use bounce, or learn to use FEC. I would immerse myself into youtube. There are tons of great videos on weddings. Can't beat the price. Main thing to remember is that you are the storyteller. Simplify it -
Chapter 1 - prepare gear- Camera(s), lenses, extra batteries, charger, flashes, extra memory card(s), extra batteries for flashes, possibly a tripod for formal shots or if shooting from back of church or do you need one?
Chapter 2 - meet with B&G, gather notes of as many details that you think you need, how will wedding flow and from and to different locations if applicable. At his point you need a "shot sheet" (many versions are downloadable free on the net). Besides some of the standard shots, what do B&G have in mind? This may expose some limitations as a first time photographer. Deal with it. If you want to do the best you can, you overcome the obstacles. If you cannot, let them know at this point and maybe perform as a second shooter to a pro, but you can do this. Shoot everyone at the event. Easier to delete too many than to explain ones you missed afterwards.
Chapter 3. - Get ready shots - B&G - This is the brides day - groom is just a figurehead, but you know this. lol
Chapter 4. Arrival at church, the ceremony, any formal shots either before or after ceremony, the exit of B&G from church. (modify as needed)
Chapter 5, The reception - B&G entry, toasts, first dance, Father/Mother dances with B&G - maybe grandparents - shoot them all. Throwing of bouquet,and removing garter, dancing by guests, Table shots of guests, lots of candid shots, B&G leaving reception.
Chapter 6 - Post-processing. Send a few shots to the phones of the B&G as soon as possible. Editing - Suggest shooting everything in RAW. Go through shots and delete any you are not happy with. Decide how many final shots you want to retouch. Generally 100-150 is a good place to go. You may want to convert rest to Jpegs and put on a separate flash drive or DVD for your friend. Are you planning for prints and an album or are you just giving images to B&G and let them decide which ones if any they want printed. Maybe you would like to work on this with them?
Final chapter - pour yourself a tall glass of your favorite refreshment, kick off your shoes and rejoice in your first effort, you deserve it!

Although not inclusive, hopefully this will get you started. There is a ton of free info out there on weddings. Take advantage of it. Maybe take B&G out for an informal shoot a couple of weeks before the wedding. Shoot in the sun, in the shade,and indoors. Get creative on this. Get comfortable doing what you do. Have fun.
You got this!!

you say the occasion is very informal and very casual, you have obviously taken some great pictures of other family outing, or they wouldn't be asking you, so it might help if you treat it like your other family outing, only focus on the happy couple, if nothing else it might help you to relax. It sounds like you are fairly close to your niece so you need to be aware you are going to be capturing her wedding on film so you won't have time to really experience and enjoy the moments, if it were me and I could afford to I would hire a photographer as a wedding present, that way I could really experience the event, just food for thought.

There are a lot of different answers to your questions. Mine are not the final word. But. . .

I would mount the 24-105. My usual goto lens for events like this is an 18-200. The 24-105 is the better range. The 70-200 will not let you broaden the shot for groups unless you stand back. In any event I've shot, I find myself shooting a table of folk and then almost immediately going for a closeup of one person or a two person conversation. The 105 will handle that situation, the 70 is just too tight. (If you have to back off, someone will almost certainly, though inadvertently, get in the way.)

One of the things I like about my a6300 is that I can set it to shoot silently. I can shoot in burst mode and often do for something like an event--not the high-speed 11fps but the middle setting of about 6fps. That fraction of a second will sometimes capture a fleeting expression that is priceless. I don't know the Canon lineup. Nikon has not figured out how to quiet the mirror. In fact both of them have dropped the ball for mirrorless.

Use a flash, but put some light modifier on it. This can be a small camera mount softbox, a Gary Fong Lightsphere, or a Light Genius. Anything that will soften the light, but better if it also emulates bounce flash. Choose your flash exposure to provide fill light, not the main light. You don't want to overcome the sun, just provide definition and soften shadows. This will soften the shadows to overcome the problem of overhead sun. (I've come to appreciate the Light Genius, but even a white handkerchief is helpful.) I used the GF lightsphere for years, but in the last six months have been using the Light Genius and like it better.

Go over the ceremony with the bride and groom. Create a shooting script. Try to know what and where your next shot is. In the thick of the action, it is very easy to get lost. Of course, make sure you have enough space on your memory card. Better yet, copy all the older images onto your computer, then reformat the card. Changing a card as the shot you were anticipating disappears is very frustrating. Having one fail is even more so. Same with batteries. Make sure they are fully charged--and bring a spare. Also, spares for the flash.

Make sure to focus on the eyes. I like to use shallow depth of field to keep viewer attention on the subject I want them to see.

Don't be shy about "'chimping". Sure, the pros and the purists will poo-poo that. But there is no better way to be sure you are actually getting something worthwhile.

A week or more before the event, set your rig up, get a couple volunteers to stand in and try it, then practice a little, especially the main poses.

I haven't shot a wedding in many years--though it helped pay the bills during grad school--so there are surely many pros out there with better advice. I also am no longer shooting Canick cameras, so YMMV (Your Mileage May Vary)

Good luck.