It's one thing to have a nice camera it's another to be able to use it especially to video a wedding. I've been doing video for a number of years but I don't feel confident enough to film a wedding. Have a look at what people who do this sort of thing for a living, have produced to get some ideas as to how to shoot it.
I hope your brother has a great day and you make a good recording of it for him. BUT do your research first.