The Ice Twins by S:K.Tremayne
I am Kirstie, I am Lydia, I lived, I died. Or did I?
After one of their identical twin daughters, Lydia, died in an accident, Angus and Sarah Moorcroft move to a remote Scottish island, hoping to mend their shattered lives. But when their surviving child, Kirstie, claims that they have mistaken her identity – that she, in fact is Lydia – their world comes crashing back down.
They know that one of their daughters died. But can they be sure which one?