Why aren't they? Well, because they don't matter to the story. This is a book based on actual rabbit behavior, and real rabbits probably don't care too much about their siblings in the long-run. Hazel and Fiver strike me as an exception, not a rule, and their bond seems to be predicated on Hazel being the only other rabbit who's realized that Fiver usually knows what he's talking about. Actually, that goes hand-in-hand with Hazels' trademark open-mindedness and ability to adopt new ideas relatively easily.
Would be interesting to see how other rabbit sibling relationship play out once they're adults. I can't actually think of any siblings aside from Hazel and Fiver in any version of WD. Can't think of any from Sandleford, or Efrafa, or Cowslip's Warren, or Redstone, or Buttercup's Warren, or anywhere. I'm racking my brain here, and I can't think of any. We get Hazel and Primrose's kits in the series, but they stay children forever. The rabbits must have siblings, in fact, I'd be surprised if any of the WD rabbits were only-kits. That's just not how rabbit breeding goes normally. So, yeah, I'd definitely say that the three missing siblings just aren't really part of Hazel or Fiver's life at all, not any more than any other Sandleford rabbit. Come to think of it, the very reason Hazel remembers having four siblings at all might just be because Fiver is named for being hrair, just over four.
Huh. Never thought about this before, but this is actually a pretty interesting quirk of rabbit life, intentional or otherwise. Can anyone confirm with me whether or not Hazel and Fiver are the only adult siblings with a bond mentioned in the whole franchise?