People wish they have sisters when they don't already have to deal with one.
also westermarck effect.
for the unenlightened (copypasta'd off wikipedo):
Westermarck effect
Reverse sexual imprinting is also seen: 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. This phenomenon, known as the Westermarck effect, was first formally described by Finnish anthropologist Edvard Westermarck. The Westermarck effect has since been observed in many places and cultures, including in the Israeli kibbutz system, and the Chinese Shim-pua marriage customs, as well as in biological-related families.
In the case of the Israeli kibbutzim (collective farms), children were reared somewhat communally in peer groups—based on age, not biological relation. A study of the marriage patterns of these children later in life revealed that out of the nearly 3,000 marriages that occurred across the kibbutz system, only fourteen were between children from the same peer group. Of those fourteen, none had been reared together during the first six years of life. This result provides evidence not only that the Westermarck effect is demonstrable, but that it operates during the critical period from birth to the age of six.[4]
When close proximity during this critical period does not occur—for example, where a brother and sister are brought up separately, never meeting one another—they may find one another highly sexually attractive when they meet as adults.[citation needed] This phenomenon is known as genetic sexual attraction. This observation is consistent with the hypothesis that the Westermarck effect evolved because it suppressed inbreeding. This attraction may also be seen with cousin couples.
