May I suggest your sister contact the breeder of her tiny and find out if the parents are still being bred. Perhaps she can get a younger sister or brother of her tiny, if there is a tiny in a future litter.
Just a thought. That's what I would do. Being the mom of a 2.2 lb tiny (5 months old, I'm hoping she reachs 3.5 or 4 lbs, but the vet is not very optimistic at this point.) Zoe will be spayed as soon as our vet is absolutely sure it's safe.