Task: Question Answering
Input (English): Passage: Zac is a lonely, highly strung city trader on the edge of a psychological breakdown. He has lost everything—his job, his girlfriend Eva (Sophia Di Martino from "Flowers") and, most devastatingly, his weird and wayward younger sister Alice, the only family he had left. Alice is now a missing person, having disappeared on a narrow boat trip along with her kindred drifter and boyfriend Toby. Zac becomes increasingly frustrated with the futile attempts of the police to find them and, eventually, decides to take matters into his own inexpert hands by starting a terribly executed video blog and scouring the dark canals of the UK in a desperate, perhaps even deluded search for clues. Struggling for information and fast losing hope, Zac reflects on his past and the difficult relationship he had with Alice. Wracked with guilt and regret, his sanity starts to unravel as he fights with memories of her in the weeks leading up to her disappearance. As he remembers her sweetly burgeoning relationship with the mysterious Toby, however, he begins to wonder if there may in fact be a grander, wilder, much stranger explanation for their disappearance. 
Question: Who did Zac's sister have a difficult relationship with?
Output (English): Zac.