Regalia: The Three Sacred Stars

Twelve years ago, in the country of Rimguard, a big incident left an unsolved mystery that has begun to fade from people's memories. Time passes, and sisters Yui and Rena are living peacefully in the Enastria Empire. However, one day, a large mecha attacks Enastria. This day marks the turning point when the two girls get caught in the vortex of fate.

(Source: Anime News Network)