You might want to consider changing your iPhone X passcode to something unique if you want to insure complete control over your new phone around your family. As evidenced by several reports of siblings, half-siblings and other close family members being able to “trick” Face ID, this is the best way to lessen the odds of it happening. Why do you ask? Because knowing the correct passcode to unlock the phone is the key to training Face ID. The X’s neural network that governs Face ID only acknowledges facial input for machine learning upon a failed unlock attempt after a subsequent successful passcode unlock. It also learns any changes to your face over time after successful unlocks. If you are letting someone else you are related to unlock your phone via passcode, there is a possibility that they will be able to train their way into unlocking your X via Face ID if you look enough alike.