Penny, this is Sheldon's TWIN sister.
Missy, this is our neighbor Penny.
The Big Bang Theory Season 1, Episode 15
"Made up my mind to date both of them
by pretending to be TWIN brothers."
The Big Bang Theory Season 9, Episode 14
but when they're TWIN primes,
The Big Bang Theory Season 9, Episode 12
...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