I am sure many reading this will be asking you to use the search feature and find those answers before posting here...
Nevertheless, here's my take. 600d is good enough and 18-135 should give you sufficient coverage for different type of pictures. Couple of things to note if I am using the equipment that you are having (I assume you are not shooting auto or program mode):
1. open your aperture as wide as possible especially indoor (i reckon it is the actual wedding day we are talking about and not the pre-wedding outdoor shoot).
2. you will need higher ISO for indoor depending on the lighting condition. I believe anything 800 and above might be needed to get a reasonable good picture.
3. pop-up your built-in flash but I doubt it would give you the effects that you want. I would use it as fill in light for table to table shots. Other candid and spontaneous pics, I would just avoid the flash but more on higher iso.
4. watch out for white balance as it depends greatly on the lighting in different restaurants and function rooms. In short, play around with the different WB mode if AWB is giving you weird color as you preview the pics.
5. oh... all the above is on the assumption you are using aperture priority mode or av. If you are using shutter priority or manual mode, I pretty much say practice, practice, practice before trying. It is a little bit more challenging unless you are very familiar with knowing what settings to change under different situation.
the above 5 pointers are more or less the basic technical setting that would work given what you have on hand...
As for other things to note, this is normally what I would do.
1. Do not irritate the main photographer... don't block, don't distract him, don't be a part of his picture.
2. Do not have to focus on what the main photographer is shooting. In fact, try to find some good frames that he is not focusing on.. surprises sometimes are in little corners.
3. You are a family member. You know more people that the photographer. Take more pictures of them because your brother and sister-in-law will be delighted and better associated with those familiar faces you are able to captured.
4. Candid pictures of children always bring joy, so do not forget them.
5. Haha.. this one is a no brainer... look up what other professional wedding photographers are capturing to get inspiration on the kind of pictures you want...
ok la.. 10 pointers ... should be enough and I believe many other members here would be offering their pointers and advice ...
Btw .. i am no pro. It has been a while since i cover my last wedding assignment. Age is catching up and actual day wedding assignment is very tiring - not to mention the amount of pre-event efforts being taken into consideration. I am keeping to shooting just 1 per year...
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Wishing you an enjoyable shoot at your brother's big day! More importantly, enjoy the wedding and be a part of your brother's joyful event. I am sure he would want that too.
Daniel