I would add a third choice:
assume it is a joke
write back as if it was a joke
something like . . 'gosh you almost had me fooled for a minute lol lol!!! Great joke!!! So what do the kids really want for Xmas?'
Make her realise that its SO absurd it just has to be a joke!!
I agree with this. If you act innocent to her true meaning you can make her see she is wrong, if you just rant at her she will just see it as you being unreasonable.
I would also add something along the lines of 'I nearly thought you were actually thinking that animals were just disposable like toys then, not real living creatures. Anyway, rabbits arent great with kids or noise or being picked up, so even if you got a tame rabbit they would probably end up being just like Freddy anyway. ' As I really do see her getting the kids a rabbit that is 'tame' to replace the bella promise, and that should warn her not to, and she might actually listen.
No offense but your sister sounds like a right idiot.