Why? cause
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(to answer your question, its because of the love connection between legit family members in a, for lack of a better word, "normal" household.
Mother loves her kids and want them to learn and grow, what better way to learn than from your Mother
Fathers want their kids to know what true love means (ill love you like your mother)
Sister loves her brother as a protector and will give herself to him in return. (not really but still)
Most men will lust over their mother/aunts or sisters first due to hormones growing up and being close to them everyday
Most women will say their father was their first love..be it forbidden love, or just "first male" love.
Its not necessarily a taboo, its just mainstreamed to be seen as taboo, in reality if two consenting adults consent to the act, everyone else has no say in what happens.)