How often has your sister used your phone? When she attempts to open the phone the first time, did you open it for her, or did she look at the phone and then enter the passcode? If she was looking at the Face ID and you provided her with the passcode, then the Face ID has learned a compilation of your face and hers through the use of the passcode. With Face ID activated, you should not allow others to open your phone with the passcode while looking at the screen. You need to reset Face ID and then activate it again 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 them look at the phone and unlock it with the passcode. That is how Face ID learns changes to your face as well.
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How often has your sister used your phone? When she attempts to open the phone the first time, did you open it for her, or did she look at the phone and then enter the passcode? If she was looking at the Face ID and you provided her with the passcode, then the Face ID has learned a compilation of your face and hers through the use of the passcode. With Face ID activated, you should not allow others to open your phone with the passcode while looking at the screen. You need to reset Face ID and then activate it again 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 them look at the phone and unlock it with the passcode. That is how Face ID learns changes to your face as well.
why can my sister unlock my phone with face recognition? (we’re not identical in any way)