That is fascinating. I had never heard of that! Link to the AMA.

Fun fact…. I’m actually an Identical MIRROR twin, the left half of my face is identical to the right half of her face. I’m right handed, she’s left handed. The right corner of her lips slightly dip while the left corner of mine slightly dips. My left eye is my good eye and her right eye is her good eye. Our natural hairlines fall on opposite sides. When we write or eat across from each other, it looks like we are each other’s reflection.

(We did a reddit AMA about this a couple years ago. Wanderingballoon is my screen name if you want to look it up.)

So technically, if my sister was the one to reverse in front of me, like this example you just gave, it would only prove to show me ‘her’ face as the reflection on the mirror and my actual face on hers since she is my reflection.

It’s always very Trippy looking at pictures of us- I almost always say she is me and I am her… when I see my ‘real’ face from a photo, it looks like my sister, because my face is her face backwards. And I’m so used to seeing her face on a daily basis, I naturally think in my mind she looks exactly like me, so when I see my own ‘different’ face in a photo, it’s hard to accept that it is really me.

Next, you have your twin walk forward to stand next to the table, and TURN AROUND so you can see his/her face next to yours. Now you see one face next to a horizontally reversed copy of itself.

QUESTION: Which is the reversed face? ANSWER: Your twin’s, since he/she turned around.

Our normal mode for observing faces is to see it reversed; that is, the right side of the face to our left, and the left side to our right. The mirror doesn’t reverse it, it leaves it the same, which is contrary to how we normally observe faces. If you always walked on the ceiling, and saw everybody else’s face upside down as your norm, you’d think a mirror inverted your face vertically, and not horizontally.