Momoyama kyodai manga

Momoyama Kyo-dai summary:

From Aerandria Scans: Arima and his little sister Chihiro are the twins of the Momoyama family. Six years ago, they've overheard their parents say "We will tell them the truth when they turn 16." The two worried that they might be adopted during those six years, but the truth they learn on their 16th birthday is even more shocking! Also includes an award-winning one-shot.