Aldnoah.Zero Gaiden: TWIN GEMINI

Twin siblings Mizuki and Haruya Mikage had a happy life, even in those tough times, but that would change when Libitina and Libertina, two twin Viscountesses from the Vers Empire, attack Sapporo, the city in which they live.
This story happens around the same time Slaine and Trillram went down to New Awara where Inaho and the others were.