1) Little Beam is from a 2e supplement book, Glories of the Most High: The Unconqeured Sun. He might be Sol's son, he might be a charlatan, but it's a dangerous claim to make if it's false and he uses it to spend most of his time in Creation trying to help the returning Solars, so he's either telling the truth or he's a liar who's totally unconcerned about Bronze Faction Sidereals.
2) Both from another 2e book, Manual of Exalted Power: Sidereals. They're essentially 'what-if?'s, the Little Sister is a sixth Maiden the others apparently made to arbitrate decisions sometimes, and Nox is their brother who represented possibilities and probability as the flipside of the deterministic Fate of the Maidens, but he sided with the Primordials in the war and had to be wrestled into submission, and is now basically lobotomised and hooked up to the Loom of Fate to keep it running. There's a fan-made supplement written about him called the Nocturnals, you can probably find it floating around somewhere, they're pretty cool, but you can also just ignore him if he's not useful for your game.
3) He's a little aside in Glories of the Most High: Luna, mentioned as a possibility. There's not much to go on, he's basically an offhand mention for people to pick up and run with if they want.
4) It's happened, they're God-Blooded, just particularly powerful (for God-Blooded).