Voice Imitation for Security
One of the secure methods of safeguarding your phone is using your voice. After all, replicating someone else's voice is no easy feat! You can also employ additional measures like Face ID for added security. However, this level of security doesn't account for siblings.
While she might not be a twin, she perfected her vocal imitation skills to the extent that she might as well be one. We hope she didn't employ this talent for any nefarious purposes.