So a simple solution would be to have our dear narrator give us a short recap of who Shar is in the Forgotten realms at an early point in the game, as she seems to play a major role. It could be a quick recap of Shar vs. Selune and describing their roles.
And it's for the PLAYER, not for the characters ingame.
This is a recurring problem with BG3: oversights that can be easily fixed with a sentence of in-game text or just a bit of spoken dialogue.
