As per filming any events, you can start by identifying the major segments of the wedding day and tackle each segment as they come. E.g. Bridal preparations in the morning, the games and gatecrashing by the groom when he comes and fetch the bride, tea ceremony, church wedding, etc.
Then for each segment, identify the various smaller activities that may take place. E.g. For Bridal prep in the morning, you have the Bride's makeup, bride's parents busying getting things ready, the Jie Meis getting the bittergourd and such ready, etc.... For the Gatecrashing, you have the arrival of the bridal car, groom and groomsmen going up to the bride's home, games to play at the door and winning of the bride.
Then you can capture each of this activities when you see it happens. If this is your 1st time, don't be too bothered about getting the most creative angles. Just 1) set auto-focus, 2) frame appropriately, 3)manually set the correct exposure, and 4) hold your cam as steady as possible, 5) don't zoom in, zoom out all the time (do gradual slow zooms if you want).
I believe your friend's sister merely just want a straight forward documentary coverage, so focus on getting the content. Usually the most pleasant footage is a boring static recording of the scene.
For recording of long speeches, try to use a tripod. If you go handheld and want a stable shot, close the LCD and use the eye viewfinder (and lean your back against a wall if there is one behind you).
If you want to get good audio, you have to go close to get your camera mic close to your subject. If you want a more journalistic, non-intrusive angle, stand far away and do a full zoom (but your footage make shake a lot if the cam is not held steady)
Don't be too influenced by the online highlights done by full-timers. Those are shot with the final edit in mind, hence they know what scenes to sacrifice so that they can get into a better position for the frame they want. Since this is your 1st time, just press record and hold it steady if you are not sure.
Other than these generic advices.... just have fun lah. Oh, remember to have sufficient battery juice and tapes/flash media!
helpful