The most important focal length for a wedding is the one you need at the time. Weddings cover a lot of different shots from a lot of different distances. Shooting a wedding on prime lenses is possible but tricky and requires a lot of planning of when you can get shots without missing something else, so it will be doubly hard for an amateur/first time wedding shooter.

My personal standard load out is a 24-70 f/2.8 and 70-200 f/2.8 on a full frame camera. Dealing with your lens selections and experience level, for the ceremony I'd probably put the 70-200 on the main body and the 30mm on the backup. I'd shoot with both bodies using the main body to get close shots from a distance and use the backup for capturing the wider room shots and such. This will let you avoid lens changes and will also keep you out of the way in case anything goes wrong that you need to fix or adjust.

At the reception, I'd probably switch the 70-200 to the backup body and put the 30mm or maybe even the 10-20mm on the main body for general candid shots. I'd use a mix of the 30 and 50mm lenses in portraits.

Be sure to remember that light may be a factor when using slower lenses so be mindful of your shutter speed. Also, be mindful of the depth of field when shooting moving shots as really narrow depth of field can be hard to hit well in low light on moving subjects (for walk in and walk out for example).

Also remember that getting the shot is more important than getting the shot with the right lens. Go to the rehearsal, make sure you know where you need to be and when for key shots and don't miss shots because of badly timed lens changes.

Get a monopod if you don't have one already, particularly for when shooting with the 70-200. At those distances, in low light, camera shake will be a very real issue. The tripod isn't really needed for most of the shooting, but can be helpful for portraits if you are doing those.

Make sure you set expectations well in terms of your experience as well. A whole lot can go wrong shooting a wedding. Most of being a wedding photographer isn't about taking great photos (though you need to be able to do that), it's about dealing well with people (which you should presumably be able to do since they are family), being on your toes for changes, knowing weddings inside and out so you know where to be and when (which is experience you won't have outside of maybe a rehearsal) and being able to be out of the way while getting the shots so you are a help to the couple's day rather than an annoying distraction.