It all depends on what you have, and what you intend to do with the offspring. If the parents are unrelated and all you are trying to breed is meat for the table, a full sibling cross is fine. If one family line has some distinctive inheritable trait, breeding animals from that line together (however they may be related) increases the likelihood of that trait showing up in the offspring, and that goes for both positive and negative traits. IMO, there are no hard-and-fast "rules" about what kind of breeding is "good" and what is "bad." Realistically, when it comes to how many kits you can raise, the fact that your animals come from healthy, vigorous, productive stock is more important than how closely they are related to each other.
Thank you! We have a New Zealand buck and 4 of his babies. We plan on keeping the 3 best (which is a buck and two does) and try them. But I wasn't sure if I needed to make sure not to breed the siblings together. They are my only full bloods, I also have 2 mixed does (one Flemish/Dutch mix and one Chinchilla/California mix).