I guess there is supposed to be a Ying-Yang relationship between Shar and Selune, but let's face it, the moon is not really very bright. And what about eclipses (next one is April 8th) where the moon blocks the sun? The Shar end of it makes sense, and yes Lethe and perhaps Nyx as well are strong classical sources.
The Life domain for the moon goddess does have some precedence in ancient literature since the cycles of the moon kind of draw a parallel with ... uh, well, ahhh, you know ... biological monthly cycles. The Light domain in BGIII is very fire-oriented, with spells like Burning Hands, Fireball, and Wall of Fire, which are good for sun-gods but I don't see as a good fit for Selune. Selune clerics should have things like Moonbeam and maybe even some benevolent lycanthropy shapechange options, just like SH's parents as the OP mentioned. Moon stuff.
