Face ID opens by my brother's face every time

About Face ID advanced technology - Apple Support

"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."

Ali ~ What Apple device is the problem occurring on? ...And please add your Apple products to your Community profile:

• When signed in, you'll see "Ali2310366" at upper left. Click on it.

• In the resulting pop-up, click on "View profile".

• In the resulting page, scroll to the bottom and click on "Add a product".

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It may open the first time every time from a sibling or relative. There is nothing in the article that states it may take up to 5 times. There is no way they will ever be able to guarantee that your face is the only one that will ever open the iPhone. However, the likelihood of a random person being able to is approximately 1 in 1,000,000. That means that on the entire population of the world, there are about 7,700 people that may be able to unlock your iPhone with FaceID. The likelihood of a random person unlocking your iPhone using TouchID was much, much lower (1 in 50,000 if I remember right). Devices without a true depth camera that use a form of facial recognition are able to fool it with just a picture of the person. Apple's FaceID is much more secure.

At this point you have a couple of options. Continue to use FaceID knowing that your brother can unlock it at any time. You could re-enroll FaceID and see if it is able to keep your brother out. Or you can just use a passcode.