NEVER MIND WHAT i said!!!! NEVER MIND WHAT I SAID!!!!! @@Light Blade, I'm really sorry. I was, uh... captivated by Luna. @@Nightmare Muffin, Luna is a great princess but Twilight is an even GREATER PRINCESS! It has even been proven she's her mentor. At the end of Rainbow Rocks, Sunny wrote a letter to Twilight. Also, Sunny is learning about FRIENSHIP and Twilight is the PRINCESS OF FRIENDSHIP. Sunny doesn't even know Luna because Luna was banished at the time Sunny was Tia's student. Twilight is the ONLY PONY she has had contact with in a long time, and they've grown very close, enough for them to be mentor and student. There is no doubt that in my mind that Twilight IS Sunny's mentor, indirectly.
And you know very well why she's not in Equestria.