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...

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.

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.