Twilight and Shining Armor were never brother and sister.

Little Twilight has a crush on the most hansome, strong, and wonderful stallion in all of Equestria! Shining Armor!

A few years later, she is living in Ponyville and just recieved an invitation from the stallion himself. To what you may ask? His wedding.

Who is this Mi Amore Cadenza he's marrying, and why isn't it Twilight?

Rated Teen for language and innuendos of sex