https://techcrunch.com/2017/11/05/psa-dont-train-face-id-on-your-siblings-face-accidentally/

Some people are griping about Face ID, noting that sometimes their siblings can open their iPhone Xs using Face ID. There’s a simple reason for this.

When your sibling who kind of looks like you unsuccessfully tries to unlock your phone using their face and then you enter your password, you’re accidentally training Face ID on your sibling’s face. Therefore, if that same person tries to unlock your phone using their face, it’s possible the phone will unlock.This is a characteristic of the machine learning capabilities behind Face ID.