This project is a little different than the others, in that it doesn't directly involve your CB. Remember, sometimes there are things that will help your relationship that can only be done alone. I've given these the tag "solo" so they are easier to find.
If your CB has siblings, try to take each one of them out to visit, particlarly any of them that are old enough to want to develop a personal relationship. You can go out for icecream, a meal, a walk in the park, anything. My Knight has 7 siblings, six of them younger. One I took out to Build-A-Bear and then we got our pictures done. With another I got shakes and we went exploring in a park. Match the activity with the sibling. I've started asking where they want to go or what they want to do--they often have some fun ideas! (though I have to specify the price range first ;) )
The point of this exercise is first to develop a relationship with the rest of the family--something you will be grateful for later on--and second to show that you aren't trying to "steal away" their dearly loved sibling. I have yet to observe a courtship in which the siblings didn't feel some level of insecurity at the new addition to the family (you). By showing a personal interest in the other family members you can help head-off these insecurities.