Where did the Royal Pony Sisters come from? Are there others like them? If so, are they as wonderful and caring as the Sisters?

These are some questions that Equestrians never really thought, or cared to ask. The Sisters did everything they could to make their lives wonderful. That's all they needed to know.

For the Sisters this was a marvelous phenomenon. It made forgetting that they even had pasts at all quite an easy task.

Unfortunately, one does not always need to ask a question to get the answer.