That's often called linebreeding, which is a close form of inbreeding, and it is used by many breeders to increase the prevalence of desirable traits in their herd. If your buck has outstanding qualities you want to reproduce in your rabbits, breeding his female offspring back to him is the quickest way to do it. Just be sure it's the buck's qualities you want to show up more often! If the doe is better quality than he is, you would instead breed a male offspring back to her. If the kits are better than both of the parents, you might even breed two of them together.
Breeding siblings is also linebreeding or inbreeding, but both terms are really on a spectrum of how close breeding pairs are genetically. A sire and his offspring are exactly 50% related to each other. The same goes for a dam and her offspring. Siblings are, statistically, 50% related as well, but it's only a statistical relationship; theoretically, siblings could share 100% of their genetic material, or they could have absolutely nothing in common... 0% shared genetic material. Both of those extremes are unlikely, but it's that upper end of shared genetic material that causes concerns about sibling pairings. The worry is that harmful recessives will pair up in a sibling x sibling mating. But that is also possible with sire x offspring or dam x offspring matings, in which you know
for sure that the animals share 50% of their genetic material (or more if they come from an already linebred population). On that note, breeding two rabbits that are not related at all can
still result in negative results; in fact, most troubles I've had in my herd have resulted from bringing in an unrelated animal, which carried in problems I had never seen in my herd before that (malocclusion in our Polish, for example).
The opposite side of the coin is that in a sibling x sibling mating, desireable quailities that are present in both animals can be "set" in their offspring. For example, breeding a buck that had tremendous depth with a distantly related doe of excellent width produced a litter of kits that nearly all had tremendous depth and width. Breeding the female kits back to the sire would possibly give the resulting kits good depth; breeding a male kit back to the doe could produce kits with good width. But breeding the kits with each other could result in litters of consistently good depth
and width. I have used sibling x sibling pairings that produced numerous grand champions, and ended up being the foundations of several of my current genetic lines. The original sib x sib breeding was in 2012, meaning that line has produced excellent, healthy, winning rabbits for over a decade.
The moral is that whatever traits the breeding pair have in common due to shared genetics will be most likely to show up more frequently in their offspring. That's exactly why people linbreed - they want all of their rabbits to have the same desirable traits. The trick is to choose your breeding pairs with this in mind; be sure to include not only traits like body type and color, but also temperament and health.
Thee have been some good discussion on this forum about this subject. Here is a recent one:
https://rabbittalk.com/threads/breeding-an-outcross-to-a-littermate.37059/