There seems to be much sibling love between the Cuoco girls. However, during a shared scene on "The Big Bang Theory," the characters aren't exactly best friends.

In Season 6, Episode 16 ("The Tangible Affection Proof"), the group is struggling to enjoy the perfect Valentine's Day. Sheldon (Jim Parsons) is clueless about what gift to give Amy (Mayim Bialik), Howard (Simon Helberg) and Amy (Melissa Rauch) are in a tiff over laundry, and Raj (Kunal Nayyar) and Stuart (Kevin Sussman) plan a party for fellow singles.

Meanwhile, Leonard (Johnny Galecki) is trying to give Penny the perfect date night, but during their romantic dinner, she spots her old flame and the woman he cheated on her with — Gretchen, played by Briana Cuoco. Penny, who hates the fact that Gretchen lost weight and looks good, is livid when her ex proposes at the restaurant.