Complex sentence: In 1891, Helene, the younger sister of Emilie, married Ernst Klimt, the brother of Gustav Klimt, but when Ernst died in December 1892, Gustav was made Helene's guardian.
Split into simpler sentences: In 1891, Helene, the younger sister of Emilie, married Ernst Klimt, the brother of Gustav Klimt. When Ernst died in December 1892, Gustav was made Helene's guardian.