Face ID problem

Hello, zegaryar. Thank you for posting your question to the Apple Support Communities. You have come to the right place to get help with using the Face ID features of your iPhone. We understand your concern and how you are noticing that your brother is able to unlock your device using Face ID. We are happy to explain why this may occur, and what you can do to secure your device. 

To get started, we would like to provide you with an Apple Support article that has details about the technology used with Face ID. Please read over this information first: 

About Face ID advanced technology - Apple Support

According to the article above: 

"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.”

With the above information in mind, it may be possible for your brother to unlock your device using Face ID. If you would like to avoid this and secure your device further, we can suggest that you select to use only a passcode to unlock your device. 

To turn Unlock with Face ID on or off, go to Settings > Face ID & Passcode, then tap iPhone Unlock. You can find more information about managing your Face ID settings in this last article: 

Use Face ID on your iPhone or iPad Pro - Apple Support

We hope that the information in our reply here helps you understand more about Face ID and how to secure your device with the options that work best for you. If you have any additional questions or concerns, please reply back here to continue. 

Have a great day!