Honest question, when you say "sibling", do you mean an identical twin? Because if yes, then fingerprint sensor won't save you from that either.
After googling for a bit, I think I found what you have been referencing. Turns out, FaceID learns as it goes, even from non-successful attempts. So if every time your non-identical-twin sister tries to unlock your phone using FaceID, fails, and then you immediately unlock it for her with your face or password, then the system will slowly over time learn to recognize her face as well. This makes sense to me.
