Face id irruption

Apple has stated that siblings have a good chance of being able unlock a phone.

If you dont trust your family, use a passcode instead.

https://support.apple.com/en-us/HT208108

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.

Are you and your brother twins? Do you look very muck alike? If that is the case then it is one reason why his face unlocks your devices activated with Face ID.

The other way that your brother can unlock your devices using Face ID is if you or he added his face scan to your devices as well. Your iPhone and iPad can hold an alternate Face information as well and that means that you can have two faces stored into the Face ID chip of each device.

Axel F.