But yes they are; b is much the more common in most contexts, but in one like this a would be possible:
"Do you come from a big family?"
"Well, I don't have sisters, but I do have three brothers."
Even here, an "any" would be likely; but it would be acceptable to omit it. (In fact, I can't think of a context where "any" wouldn't be right, so if in doubt include it.... (I think that's safe advice - could another teacher confirm this? )
You could even front it:
"Want some chocolate? ... Take the whole bar."
"I couldn't - that's far too much for just me."
"OK, share it with your sisters."
"Sisters I don't have; only brothers."
(This sort of fronting is quite common - see Swan, 513).