The Tree of Hands - Ruth Rendell

Once when Benet was about fourteen they had been alone in a train carriage together – and Mopsa had tried to stab her with a carving knife.

It was now some time since Benet had seen her mad mother. So when Mopsa arrived at the airport looking drab and colourless in a dowdy grey suit, Benet tried not to hate her.

But then the tragic death of a child begins a chain of deception, kidnap and murder.