Fanon: the truth of sweetie bell.

I believe that sweetie bell is the illegitimate child of rarity and diamond tiara's father.

proof: rarity raises her alone and is considered a high society pony, even though she lives in ponyville and rarely attends high society events.
Also, swetie bell and diamond tiara are very similar in appearance. Their colors are also nearly the same.
finally, a blank flank is not a strong enough reason for her level of bullying. I think that she just wanted an excuse to make fun of her father's "bastard"