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.
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.