I have been in your shoes - I shot my brother's wedding several years ago. I had done a few before, and I've done about a dozen since. But man - you really have your work cut out for you.
Here are some practical tips based on your specific situation:
1) Like it has been suggested, strongly consider hiring a professional photographer as a gift to your brother and his wife (and to yourself). I get the sense you're not going to take this advice and will shoot it anyway, which I understand. But yeah, you will not experience the wedding from behind the camera. My memories of my brother's wedding are fuzzy and hectic. It's a shame really.
2) Eat before you arrive and bring snacks. You might get to sit down and have a meal, but do not count on it. You might have all the best equipment in the world, but if you're famished, your work will greatly suffer. Eat and drink (water) at every chance you get, because you won't get many. And honestly, you should use any breaks you have to review your images to make sure you're actually capturing what you think you are. If your lens got switched to manual focus and you didn't realize it, that is one of the most frustrating experiences you can imagine.
3) If you are struggling with low light (which you will, or worse - you won't realize that you are), put the camera on auto, put the flash on your camera and point it straight at your subject and simply focus on putting yourself in the best position possible. Your photos will definitely not look "awesome," but they will be properly exposed and in focus. And because you aren't fiddling with your gear, you can get in places and take photos that none of the guests will, which will set your photos apart from the ones that they'll post on Facebook immediately after (and during) the wedding.
4) Don't plan on switching lenses on camera bodies at any point during the day. Have 2 cameras and 2 lenses (extras in your bag for backup are fine). You'll miss photos when switching lenses, and you'll be rushed, likely dropping things. With your gear, I'd put the 24-70 on the T3i and the 50mm on the 60D. This leaves you without a telephoto lens, so if you can get a 70-200, use that instead of the 50. But don't sweat it if you can't get a 70-200. I love mine and use it a lot, but for weddings, I usually go more wide.
5) Put the priority on capturing the moment vs. making it look pretty. When it comes down to it, just get the photo. And then get lots more.
This is not advice I'd give to anyone who wants to deliver the most professional, aesthetically-pleasing wedding photos, but it is practical advice aimed at simply not screwing up majorly. Good luck, and let us know how it turns out!