S02:E25+E26 - A Canterlot Wedding

Episode 25:

Title: A Canterlot Wedding (Part 1)
Air Date: April 21, 2012
Synopsis: Twilight is confused by her conflicting feelings when she finds out her brother Shining Armor is marrying Celestia's niece Mi Amore.

Episode 26:

Title: A Canterlot Wedding (Part 2)
Air Date: April 21, 2012
Synopsis: Twilight saves her brother and all of ponykind by freeing the real Mi Amore and defeating Queen Chrysalis, a changeling that had assumed Mi Amore's appearance in an attempt to take over Equestria.