You will need to use Disk Utility to create an image (DMG, but the extension may actually be img or cdr) file of the disc you want to copy.
Load the DVD and bring up Disk Utility.
Select the DVD drive in the volumes and drives window, then click the "New Image+" icon.
You will get a Save As... dialog with some options. You should select "CD/DVD master" in the "Image Format" pop-up menu and "none" for the "Encryption" popup.
Name the file and select the directory you want to save it in, then click the "Save" button and you should then see a progress window in the finder as the image file is created. Note that you will need at least as much free disk space as the size of the disk you are imaging. The disk image file will be shown below the line in the Drives/Volumes listing.
Now, to burn a new disc, you select the image file in Disk Utility, click the burn icon, load a disc and wait for the burn dialog to appear once the disk is recognized. Make your desired speed selection and click the "Burn" button and you are on your way. Repeat until the siblings are all accounted for.