This is the most batshit crazy, soap opera-esque novel I have ever read. It revolves around multiple completely insane coincidences, and there are three unrelated women who are all deranged criminals with unhealthy relationships with their kids (and a fourth who is fixated on her kid who's slightly less deranged, and a fifth who is not a mother but fixated on her baby brother).
26 years before the book starts a homeless teenage girl gives birth to twins. Fortunately a doctor and the nurse he's having an affair with happen to be passing and help deliver the babies. Unfortunately the girl and one of the babies die. The doctor and his girlfriend bury the bodies in her back garden - which is a very normal thing for medical professionals to do when a patient passes of natural causes - and he steals the other baby and takes it home to give to his wife who is psychotic and has been pretending to be pregnant by stuffing a cushion up her shirt. Three years later the doctor and his wife die in a car accident and the child is adopted by an even more psychotic woman, who goes to extremes to make sure he spends his entire life believing she's his biological mother, and tries to kill his girlfriends so she won't lose him.
He meets a girl on holiday in Spain and they fall in love, then he discovers she's his long-lost sister (the doctor and his wife's real child) who has been fixated on finding her little brother her entire life. Then in another coincidence he lands a job as music tutor to a girl who turns out to be the granddaughter/murderer (via mercy killing) of the nurse who helped deliver him and bury his real mum's body, the one his girlfriend/sister's dad was sleeping with. He also just happens to be present when his mum and twin's bodies are discovered, and is briefly a suspect.
In the end he and his girlfriend decide to get married even though they were raised as siblings and she's been obsessed with finding him as her brother her entire life, because they aren't genetically related. Romantic!
It's a fun, quick read, but basically the literary equivalent of watching an episode of Hollyoaks or Days of Our Lives.