The Westermarck Effect is a psychological phenomena named after Finnish anthropologist Edvard Westermarck. The effect is that (according to
): "when two people live in close domestic proximity during the first few years in the life of either one, both are desensitized to later close sexual attraction." Which is why most people don't get the hots for their sibling.
However, if siblings don't grow up together and only meet for the first time later in life, they may be intensely sexually attracted to each other. This is known as genetic sexual attraction, or GSA. Again, from Wikipedia
Several factors may contribute to GSA. People commonly rank faces similar to their own as more attractive, trustworthy, etc. than average... Shared interests and personality traits are commonly considered desirable in a mate... In cases of parent-child attraction, the parent may recognize traits of their sometime mate in the child. Such reunions typically produce complex emotions in all involved.
Finally, there is the phenomena known as the Westermarck Trap, which occurs when two people who have grown up together (and thus are sexually desensitized to each other) are expected to marry each other, because of an arranged marriage. According to one theory
, this is what the novel Frankenstein depicts:
Students of the Westermarck effect may be interested to know that this trap is depicted in the novel Frankenstein by Mary Shelley, in which Victor Frankenstein is expected to marry a cousin reared with him. Instead, he creates a monster that persecutes him and murders his prospective bride before the marriage can be consummated. It is suggested that the plot owes something to Mary Shelley's own experience of the Westermarck effect, following a childhood in which she was reared with a stepbrother. Her own personal solution was not to create a monster but to elope with a married man (Percy Bysshe Shelley) at the age of 16.