muguy is absolutely correct, although the statistics are still stacked against it occurring.
I tend to be more “suspicious” and suggest the following to ascertain for sure.
First: Does your sister know your device’s passcode? If so, change it.
Second: Is Autolock set to “Never?” If so, change it.
Third: Reset your FaceID … adding ONLY your face as BOTH appearances. (this prevents someone else from adding their face later; without 1st deleting yours)
Lastly, see if your sister can still unlock your phone.
If she can, you’re indeed a statitistical anomaly and should consider NOT using FaceID while in the vicinity of your sister.