It is possible that Face ID might unlock the device for you and your sister if the two of you have similar looks. Face ID continuously learns how to grant you. It’s doing this each time you are prompted for a passcode and the 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 acknowledge her face too.
Why can my sister unlock my Face ID?
With Face ID activated, you should not permit others to open your phone using the passcode at the same time looking at the screen. You need to reset your Face ID and then reactivate it with your face.
Then if someone else wishes to use your device, you need to unlock it and hand it to them, and not let the person look at the phone and unlock it with the passcode.