My younger brother can unlock my iPhone X using Face ID

From : About Face ID advanced technology - Apple Support

The probability that a random person in the population could look at your iPhone X and unlock it using Face ID is approximately 1 in 1,000,000 (versus 1 in 50,000 for Touch ID). 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.

My brother is 8 years older than me and he can unlock my iPhone Face ID, on every try. That's wild. My younger brother cannot and he's only 3 years younger.