My sister can unlock my Face ID and we are not twins

VeraKholodilova wrote:

How can I send video for you?

What for? You want to show me how you and your sister can unlock your iPhone? There is no need, and I do not need to see it.

Change passcode on your iPhone and don't give it to your sister. Then reset Face ID configuration and setup again. When done, see if your sister can unlock your iPhone with Face ID only without typing in the passcode.

If she still can unlock it, then you and your sister look alike enough, so Face ID cannot distinguish difference between two of you.

In such case, If you don't trust your sister enough to leave your iPhone next to her, then turn off Face ID and use passcode only.

If you prefer to keep Face ID, while your sister is nearby, initiate power off/Emergency SOS by pressing and holding either volume button and the side button simultaneously for 2 seconds and this will disable Face ID on Lock Screen temporarily, so you need then enter your passcode to unlock your iPhone.

You can learn more about Face ID here: https://support.apple.com/en-ie/HT208108

It is possible that Face ID might unlock the device for you and your sister if two of you have similar look. Face ID constantly learns how to recognise you. It's doing this each time you are prompt for passcode and correct passcode is entered to unlock the device. So if your sister knows the passcode for your iPhone and successfully has unlocked the device, then Face ID has learned how to recognised her face too.

Reset Face ID settings and change your passcode so your sister no longer knows it. Then see if she can still unlock it.

If someone ask you if they could use your iPhone, unlock it first. You should not share your passcode. If someone else knows your passcode, then Face ID is not something you should worry too much about.