I had wanted Rhea to say "dweeb" instead of "Diane", but this was already the most self-indulgent comic in an already self-indulgent storyline without me making it even more likely people would miss the Cheers reference.
Diane, Lucy, and Rhoda are all named after sitcom characters. There's no significance to the names chosen beyond "sitcom names I was aware of", and "I know someone named Mary, so nobody's getting named after Mary Tyler Moore". As far as I can recall, I had no point to going with sitcom names beyond my own amusement, and thinking it would be a fun bit of trivia.
When the time came to name Diane's sister, I wanted to keep the theme going, but with a bit of a twist. Wanting her sister's name to have a connection to Diane's, but not wanting to be too on the nose about it, I named Rhea after the actress who played Diane's coworker, Carla, on Cheers.
Long story short, yay convoluted Cheers reference that only people who already knew the naming history would fully appreciate. WOO SELF-INDULGENCE YAAAAY