Regalia: The Three Sacred Stars

Twelve years ago, in the country of Rimguard, a major incident left behind an unsolved mystery that has slowly begun to fade from people's memories. As time passes, the sisters Yui and Rena are living peaceful lives in the Enastria Empire. But one day, a giant mecha attacks Enastria. That day becomes the turning point when the two girls are caught in the vortex of fate.