Nothing wrong with the brother shooting the sister's wedding. They have their own reasons.
I suggest renting lenses, maybe even an extra body. Unless you trust the second shooter.
Id rent a 35L for good portraits and general photos, and a 70-200 2.8L II. Maybe Id through a 16-35L in the mix for the second camera/shooter.
These are what I would use based on my creativity for shots.


