The Sakamaki Step Sister

The Sakamaki's had a deep dark secret hidden, including karlheinz himself. No one knew this secret but only them , until yui found a hidden room in the basement that held a single bed , a nightstand, some flowers, and a huge frame picture above the bed showing a little girl with long purple hair with greenish yellowish eyes. There was something off about it that yui couldn't quite figure it out, so what she did led her to an uncomfortable and unfortunate tale that the brothers had to tell.