His 1965 monograph about Georg Trakl, which set a standard for his understanding and the hermeneutic approach, is worth reading even today. Some of its key sentences are quoted here. This applies particularly to his view of the relationship of the siblings Grete and Georg.
Trakl's sister Grete is an interesting, yes unique phenomenon; the dark luster of her gender illuminated – or eclipsed - the soul of the poet. She is a main actress on the stage of his life and in his imagination, he has transformed her into a legendary figure, he mythicizes her and hid their demoniacal sensuality in poetic simile, while he merged himself and her into a 2-natured being that is purified by all things sexual: youth and youthtress, stranger and strangeress, monk and monkess. Up to death he has “held her loyalty" - still the last poems call her, invoke her. When he saw himself abandoned by his God, on whom he had reflected, had sung after all his life, when this God dissolved into nothing in the Galic slaughter house, the sister remained for him the only meaningful reality. No fiction stands at Trakl's end, but a person!
Trakl took the sister into the mirror-world of his poems, he transformed her into himself; it is a kind of unio mystica which connects the two, embodied, carnal and at the same time spiritual. The physical oneness, carried out in the act of incest, is without deeper importance here because it can never be as complete and satisfying as the mental one.
Anyway there was no verifiable woman in Trakl's life who was erotically dominant (body-soul) - Grete excluded. Already by school-time the sister is for him "the most beautiful girl, the greatest artist, the rarest woman", as Bruckbauer tells; he spoke about her always "praisingly out of an internal necessity". Also his youthful friends reported agree that he advocated the glorification of incest in Wagner's "Walküre" unambiguously.