Facial recognition

https://support.apple.com/en-us/HT208108

The probability that a random person in the population could look at your iPhone or iPad Pro and unlock it using Face ID is approximately 1 in 1,000,000 with a single enrolled appearance. As an additional protection, Face ID allows only five unsuccessful match attempts before a passcode is required. The statistical probability is different for twins and siblings that look like you and among children under the age of 13, because their distinct facial features may not have fully developed. If you're concerned about this, we recommend using a passcode to authenticate.

If you let your sister try Face ID, the phone doesn’t accept it, you or she then enters the phone passcode, you just told Face ID to “learn” that face and accept it the next time around.