iPhone Unlocked with Brother's Face

The author of the post indicates that they can unlock their brother's iPhone 15 Pro using their own face. They are not twins, have a three-year age difference, and are both in their thirties. The author does not think they look much alike but supposes that this is not how Face ID is supposed to work. They wonder if this case is common and seek the opinions of other users, considering it a potential flaw in security schemes.