I believe it's
Selene, the Moon.
Struck sister, shattered, since settled.
Believed to have formed after a large object hit the Earth, throwing a large amount of material into space, which then collected into the satellite we know and love today.
Simply seen, spreading sunshine,
The Moon can be seen with the naked eye - it's pretty obvious! - but its light is all reflected sunlight.
separated, seldom sojourned,
It's a long way away and people rarely (but not never!) walk on it.
secluded side shady (supposedly).
The proverbial Dark Side of the Moon (which isn't actually dark - it's brightly lit at new moon - but we never get to see it, hence 'supposedly'). That's the clue that gave me this!