In a footnote in the beginning of Richard Adams' novel, Watership Down, he makes this statement:
Rabbits can count up to four. Any number above four is hrair “a lot.” or “a thousand.” Thus they say U Hrair-“the thousand’- to mean, collectively, all the enemies (or elil, as they call them) of rabbits- fox, stoat, weasel, cat,owl, man, etc. There were probably more than five rabbits in the litter when Fiver was born, but his name, Hrairoo, means “Little Thousand”- i.e., the little one of a lot or, as they say of pigs, “the runt.”
He explains the meaning behind Fiver's name and gives the reader a glimpse into the rabbits' language and culture. But the part that I'm most interested in is "There were probably more than five rabbits in the litter when Fiver was born...", which implies that Hazel is only one of his many siblings.
With Fiver knowing about the impending doom of the Sandleford warren and Hazel putting enough faith in him to help gather other rabbits in a blind escape, why didn't they search out some of their own siblings as well? Without disregarding the unspoken possibility of their kin leaving the warren or being taken by "The Thousand", does Richard Adams (inside or outside his novel's universe) ever explain why Hazel and Fiver would behave this way?