Sharess is Bast. She's one of the few surviving deities of the Mulhorand pantheon. She was Best originally, but changed over time as she started to become worshipped by the rest of Faerun.
That's true and what @Chupricon has said is also true. There were conflicting stories that got retconned into: Bast after she fell under Shar's spell and then freed herself.
The first version I read in 2nd edition is that she was an aspect of Shar that was worshiped by ladies of the night and anyone who had to hide their passion under a cloak of darkness and secrecy. During the time of troubles Sharess was separated from Shar. Bast, Sharresss and some minor elven goddess of dancing and revelry came together, combined their portfolios and become a single goddess. Now her temples and festhalls double as cat sanctuaries.
This is all speculation -- but in Greenwood's supplements festhalls play a larger role than they do for any other WotC author. I think he had vision where Sharess' festhalls would form a nexus of spying / information sharing. Essentially doing the same thing that the Harpers were doing but operating independently: adventurers could show up and get clues to further their quest. My guess is that others inside WotC didn't feel comfortable with the festhall intrigue plots and decided to emphasize her domain over cats.
