Well quite frankly, most likely nothing will happen. There was a study done where they did sibling(brotherxsister) crosses for 18 generations before starting to have problems. Rabbits can handle quite a bit of tight inbreeding, although I personally do not like full sibling crosses because you tend to get very good rabbits or very poor rabbits quality wise. Every domestic breed of animals has been created using inbreeding and linebreeding, its practiced by many many breeders, as its the quickest way to fix traits. Also a great deal of inbreeding goes on in the wild, for instance all cheetahs are so closely related that you can do organ transplants or blood tranfusions from any cheetah in the world to another cheetah. Sadly that also means their at a genetic bottleneck.
But what you have to ask yourself is why am I inbreeding? Are the rabbits exceptional quality rabbits that your trying to set certain traits? Or are you just going to cross these rabbits because its all that you have? Thats not a good reason to inbreed, because you'll be doubling up on the bad as well as the good qualities, and the bad may be more dominate. You can do it, but you need excellant stock, a plan and a goal for the future. If thats not the case don't do it.