This study aims to give more understanding about incestuous behaviour especially in Virginia Cleo Andrews’ Flowers in the Attic and Tabitha Suzuma’s Forbidden. There are three focuses of the study; they are (1) how incest described in those novels, (2) what the causing factors of incest, and (3) what the impact of it. In answering the first question, incest on its features can be classified into three types; they are based on the offenders, the motivation, and the way of treating. For the second question, this study is analysed trough two parts. The first part is based on the condition before that incest happened and for the second part is based on the psychoanalysis theory by Karen Horney. In her theory, she explains about the childhood experience and neurotic needs that are related in finding the possible causing factors of incest as the focus of this study. Then, for the third question, the possible effects of sibling incest such as; trauma, isolated on social sanction, sexual aversion disorder, and other psychological problems. The result of this study has been found that sibling incest which is happened in those novels is driven by both sides of the offenders. What they do are on their mutual desire and willingness. And the dysfunctional family that they face also really influences their psychology. In Forbidden, it is found that there are two motivations that influence Lochan and Maya to do sibling incest, they are; affection and aggression while in Flowers in the Attic, there are three motivations that are found, they are; affection, eroticism and aggression. For the second question, this research reveals that there are three causing factors of sibling incest that are happened in both novels, they are; dysfunctional family, between age peers, and law of homogamy. Not only that, from the findings, it is also found that all of the offenders experienced the child abuse and neglect in their childhood. They also get the emotional maltreatment from their parents. Their childhood experiences then shape their personality and it deals with the psychological problems which are influenced by the neurotic needs that they seek. The third question is about the impact of sibling incest towards the offenders’ life. And it is found that all of the offenders blame themselves for everything that has happened in their lives, even one of those offenders Lochan decides to commit suicide