If Princess Celestia rules over everything. Who is the queen? Or King.... If there are multiple princesses Does that mean that Twilight Sparkle is related to Celestia and Nightmare Moon?

Princess Celestia co-rules Equestria with Princess Luna.
Princess Cadance rules the Crystal Empire. However, her husband, Shining Armor wasn't royalty, so her designation didn't rise, keeping her as "Princess."
Twilight Sparkle became Princess through decree of Celestia. She wasn't born into the role nor did she marry into it. Celestia just went "You're a Princess now. Deal with it."

this Remineds me some thing Docotr whooves mentioed in the fan audio dramas i listen two. on that note some one animated first 3 Docotor Whooves and assistant on like with the peisode audio so check it out

http://www.youtube.com/watch?v=kdhmFFKEuog

Celestia and Luna form a diarchal principality. Cadence rules her own sovereign nation and before the Crystal Empire returned, she had the same status that Twilight has now; the title of Princess but no governmental authority. Twi and Cadence were awarded their titles for what amounted to humantarianism by our standards.

I'm still waiting to see Prince Blueblood's family tree to see how in the hell he's related to Celestia.

Considering naming conventions in Equestria, he probably met the same criteria for princehood as this Prince! :lol

Or maybe he's Cadence's douchey half brother?

But Cadance isn't related to Celestia, while Rarity specified that Blueblood is.

But if he's related to that Prince, I quit :lol

If Granny Smith was alive when Ponyville was founded and Twilight said that Ponyville hasn't wrapped up Winter with Magic because of the Earth Ponies founding it 300 years ago, I think it is safe to say that Cadence could have Blueblood as a half brother while still not being related to Celestia despite that idea making science cry. :lol

They're all Apples.
There. Now it all makes sense.
Celestia's actually the first Apple.
http://www.youtube.com/watch?v=FOC7qHZ8CMY

still curious what spell does Celestia no and same goes to luna to keep them young i mean really shouldent they be like really 10000 year old a de-creped