In September, the company had claimed that there is a 1 in 1,000,000 chance of a different person being able to unlock someone else’s iPhone X with Face ID versus 1 in 50,000 with Touch ID feature. The company also states on their website that twins and siblings who look alike and are under the age of 13 can get through as their distinct facial features may not have fully developed.
In the video, the elder brother was able to unlock the phone smoothly as he had registered his facial features. As per Apples claim, the younger brother should not be able to register but surprisingly with the glasses on, the younger one was also able to unlock through the Face ID feature smoothly. The video posted by a Reddit user, mohanishs, puts a question mark on the security of this feature.
The brothers also posted an update video on Youtube, where they tried to re-register for the iPhone X's facial recognition feature through the elder brothers face. This time, the younger brother was not able to unlock it instantly but after putting the passcode a few times, he too was able to get through this lock system.
Apparently, the catch is that every time you unlock the phone using a passcode, iPhone X captures your face again and in the case of these brothers, the phone seems to have created two samples of the two faces, one of the elder brother and one of the younger brother. Thus both can unlock the phone.
Reportedly, the more times you write the passcode, the more accurate it is to recognize that it is you and not someone else. In this particular case, both the brothers entered the passcode and the iPhone X ingested the sample as training.
Face ID feature has already been proven to struggle with twins. Mashable had also uploaded a video on YouTube to check whether the iPhone X's facial recognition twin compatible. There also, iPhone X was not able to tell a difference between the facial features of twins.
Unless you have all faith on your sibling, you should stick to the rudimentary passcode option.