
Penny, this is Sheldon's TWIN sister.
Missy, this is our neighbor Penny.
The Big Bang Theory Season 1, Episode 15

I already shared a uterus
with my TWIN sister.
The Big Bang Theory Season 12, Episode 19

...and his five TWIN brothers,
you are wasting your time.
The Big Bang Theory Season 1, Episode 16

...since I beat out my TWIN sister
for the "Did It On The Potty" trophy.
The Big Bang Theory Season 8, Episode 23

She's my TWIN sister.
The Big Bang Theory Season 1, Episode 15