He was found guilty even though he didn't kill her. The one responsible for pushing her down the stairs was her brother who (it was suggested) loved her in more than a brotherly way and then fled through the front door as the one on trial came into the house and, not knowing his way around opened the cellar door before finding her on the floor at the bottom of the stairs in the hallway. He went to see if she was OK but she assumed he was her brother and backed away from him, falling through the cellar door and down the steps banging her head on the concrete floor of the basement thereby killing herself. The one on trial went down into the basement to see if she was OK and picked her up but she was dead. This is why his fingerprints were on the cellar door, his shoe prints were in the basement and her blood on his clothes.
It wasn't made clear wether or not the female officer thought the brother was guilty although I believe she had her suspicions about the verdict. The father knew that his son was responsible for his sister's death but kept quiet.
All said and done, I thought it was a rather poor episode of a fine series and dislike the female officer intensely. She is too pig-headed to listen to others, especially Mike (who I thought was excellent again), and her overall attitude is not going to win her any friends in the squad. Look forward to better episodes in future.