Dear Rarity:
If your sister is annoying you, then you ought to have her replaced with a robot. It would probably be a lot more efficient than your sister, and if she came back, you could have your robot go on a murderous rampage and kill her.
Of course, if what you want is someone to boss around and do your chores, you should really just steal Twilight's organic fax machine. If he complains, just use your natural sexiness to manipulate him into following your every command. Of course, if Twilight complains, just tell her she's not sexy enough for a fax machine anyway.
One thing you should never have to do, however, is stoop to the level of an inbred earth pony. Seriously, conspiring with Applejack? You should have just killed her in her sleep and taken your sister back. And probably Applejack's sister too. Not only would you have rid the world of an annoying inbred redneck, but you would also have double slave labor. Just like Twilight.
Your faithful Big Sister figure, Princess Celestia.
P.S.: If you want “Organized Chaos” in your room, then perhaps you should get a black hedgehog to help you control that chaos.
Dear Sweetie Belle:
If your sister is annoying you, then you ought to have her accused of being a robot. After all, the robot police are always listening. As are the thought police. And the fashion police.
The point is, we are always watching your every move. So you can accuse your sister of pretty much anything and you can screw her over almost immediately.
Just don't get any rebellious ideas, unless you need a one-way ticket to the moon.
Your one true Big Sister, Princess Celestia.
P.S.: War is Peace. Freedom is Slavery. Ignorance is Strength.
P.P.S.: I don't understand. At the beginning of your letter, you said “Today I thought I'd surprise Rarity,” but then you said “So the next day, I was ready to watch the race.” So, which day was it?!