Face Detection Security Problem

While odds of this occurring are very low, they are NOT zero. The odds are indeed higher for siblings, and even higher for younger siblings.

You might try the following when you are alone.

1) Change your device passcode to something which ONLY you know.

2) Reset your FaceID; re-installing only your face.

3) Setup a 2nd face appearance, also w/ your face. (This prevents someone else from installing their face on your phone w/o your knowledge)

4) Make sure that your Screen & Brightness auto-lock is set to a fairly short period - e.g. 3 minutes.

5) Never divulge your passcode.

Then, test to see if your brother’s face can STILL unlock your phone.

If he can - and if this ability concerns you - you might consider resetting (erasing) FaceID again, not setting it up, and using only the passcode.

The fol article - and it’s following articles - provide additional detail if you’re interested:

Touch ID and Face ID security – Apple Support (AU)