Complex sentence: He had a few brief relationships when younger, and seems to have been deeply attached to his half-niece, Geli Raubal, although the nature and extent of their relationship will never be known, it had the traits of latent "sexual dependence".
Split into simpler sentences: He had a few brief affairs when younger, and seems to have really been in love with his half-niece, Geli Raubal. Although the full extent of their relationship will never be known, it had the traits of latent "sexual dependence".