They're returning the rabbits to the previous owners. There's no further expectation of duty to either party.

The situation implies that the person selling the rabbits as pets would attempt to rehome them or care for them as pets.

There's a difference between being technically correct, and being moral. While the language used is technically correct for the situation, omitting the information about where the rabbits end up is immoral.

Clearly OP understands that if the whole truth was told, they wouldn't be getting a free source of rabbit meat...

Is it dishonest to buy a rabbit and not mention that you're planning to eat/breed/keep it as a pet?

No of course not, it's none of the seller's business. If they say "hey we're vegan please don't eat our rabbits" that's a different matter, it would be at the very least polite to respect those wishes.

Why on Earth would returning a rabbit to a farm where they eat them be any different? In this case the possibility is perfectly clear.

I think the bit about it being a "farm where they eat them" is the key point.

If the person returning it knows that, then sure. The OP implied pretty heavily that they don't know the ultimate fate of the rabbits.

As far as these people know its "A farm where they sell breed and sell rabbits as pets", not a farm that raises them for meat.

"it's none of the seller's business"

In this case, the parties are close relatives, so I would argue that their threshold of duty to disclose such facts is somewhat higher than in case of complete strangers.

I would hardly call it immoral. If the owners are curious about the ultimate fate of the rabbit, they could simply ask before returning.

I think it doesn't occur to them that the rabbit's new home is in the oven. They wouldn't think about asking

I think the expectation is that since they bought the rabbit there and the rabbit was happily living there, when they return it, they expect it to still live. They think they are returning the rabbit to its "natural" environment and will happily live on the farm.

So there is no legal duty or obligation, but morally, the expectation on both parties are definitely not the same. Otherwise they would tell them that they would cook them when they return them. The fact they are hiding this information shows that expectations are not aligned.

Yeah but it's a farm... it's not a rabbit rescue and pet store

Did you miss the part where the sister says "that [if] they want to let the rabbit loose, they can return it for free"?

That's an outright lie, since they aren't letting the rabbit loose to live on the farm -- they're eating it.

what matters is not what we think. What matters is what the girl recovering the rabbits thinks. If she thinks that if she told the truth the rabbits wouldn't be returned to her, then by not saying anything, she is lying by omission.