Freud offers an explanation based upon the Darwinian hypothesis of the primal horde in which the old father kept all the females for himself and drove away the growing sons.
This state of affairs has been observed among herds of wild cattle and horses. It generally leads to the killing of the oldest bull or stallion by the younger males.
=The Primal Horde.= Freud assumes that this must have been the usual occurrence in the primal horde. One day the sons joined hands and killed the father.
"Though the brothers had joined forces in order to overcome the father, each was the others' rival among the women. Each one wanted to have them all to himself like the father, and in the fight of each against the others the new organization would have perished. For there was no longer any one stronger than all the rest who could have successfully assumed the role of the father. Thus there was nothing left for the brothers to do, if they wished to live together, but to erect incest prohibitions, perhaps after many difficult experiments, in the course of which they may all have renounced the women whom they desired."
In other words, the incest taboo was adopted to assure peace within the family circle, a convenience measure dictated by jealousy.
=Repressed Incestuous Feelings= may at times drive one into a most objectional form of behavior. A brother who in childhood was too fond of his sister (or vice versa) may, from an unconscious desire for self-protection, adopt a hostile attitude to his sister. The more attracted he was to her the more sadistic he will appear in later years.
He may even avoid all the women who would in any way suggest his sister and in that way never feel satisfied in love, for the women who cannot possibly suggest to him his sister, lack all the fetishes which would vouchsafe him safety and eroticism.
Such a man should be analysed and made to realise the incestuous cravings which he has repressed into his unconscious. His hatred would then change into affection and in his search of a mate he would logically seek the sister image which alone would insure him sexual happiness.
I have reconciled in that way several groups of brothers and sisters who had never been able to get along after puberty, altho most of them had developed a dangerous fondness for each other before puberty.