Hey there,
As you use Face ID it starts updating your appearance and this is especially true when you enter your passcode shortly after Face ID doesn’t register. That means if someone tries to access your device with their face and you shortly then after enter your passcode, the appearance starts adapting.
Knowing that after a reset, enrolling your appearance and then seeing that this person is not able to unlock your device tells us that things are working as expected.
Please be sure this person does not know your passcode and that you don’t enter it after they have attempted to unlock your device.
When we’re talking about siblings, that changes the probability that they can unlock a device with Face ID and especially 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 instead.
See ☞ About Face ID advanced technology - Apple Support