Don't Close Your Eyes by Holly Seddon

Robin and Sarah weren't the closest of twins. They weren't even that similar. But they loved each other dearly. Until, in the cruellest of domestic twists, they were taken from one another. Now, in her early 30s, Robin lives alone. Agoraphobic and suffering from panic attacks, she spends her days pacing the rooms of her house. The rest of the time she watches, watches the street, the houses, the neighbours. Until one day, she sees something she shouldn't.