So, your sibling, who kind of looks like you, tries to unlock your phone using Face ID. He doesn’t manage, so you enter your password. All in good fun, right? No.
When you do this, your basically telling your phone that it was you and he didn’t recognize you, so you had to put in the password. Hence, the phone is going to learn its lesson and recognize your sibling as yourself the next time he tries to unlock it. It’s machine learning at its worst.
Apple clearly states this sort of dynamic in one of those documents and manuals no one has ever read: “This data will be refined and updated as you use Face ID to improve your experience, including when you successfully authenticate. Face ID will also update this data when it detects a close match but a passcode is subsequently entered to unlock the device.”
So the next time you’re testing Face ID with someone who looks like you: don’t enter the password right after it.