A Pieceful family

Katherine is 16 who had a perfect family. But is it really as it seem's.
Johan and Sarah Katherine's mother and father seemed so in love with each other but one day Sarah is found dead and johan is found holding a knife. What made him kill her?
Katherine still finds it difficult to accept the fact that Johan murdered Sarah. She decides to unfold the truth and prove her dad's innocence while her sister Samantha is convinced that their father was the one to kill Sarah.
Who was the murderer?