Sentence:  In fact, perhaps this is the pure and perfect incest: the brother realising that the sister's virginity must be destroyed in order to have existed at all, taking that virginity in the person of the brother-in-law, the man whom he would be if he could become, <WSD>metamorphose</WSD> into, the lover, the husband; by whom he would be despoiled, choose for despoiler, if the could become, <WSD>metamorphose</WSD> into the sister, the mistress, the bride. 
Meaning of the word marked with <WSD> tags:  To undergo some transformation.