Another way of thinking about it is mirrors swap what is near and far instead of rotating. If your twin was standing on the other side of glass without rotation you would see their back instead of your front. If the mirror is above you then you see your head. If you twin is standing above you then you see their feet.

Flipping thins inside out sounds strange, but it also works. If you take a mask with the same face on the inside and outside and look behind the mask you see the same thing as what you see in the mirror excluding depth perception issues.

Finally, you can think of mirrors as swapping both left and right and top to bottom. If your twin is rotated so their right hand is touching your right hand and their left hand is touching your left hand then their feet are up in the air. (Easiest to visualize with two action figures.)