Complex sentence: Löwenstein was the eldest son and heir to Alois-Konstantin, 9th Prince of Löwenstein-Wertheim-Rosenberg, who became Hereditary Prince in 1990 upon the death of his grandfather.
Split into simpler sentences: Löwenstein was the eldest son and heir to Alois-Konstantin, 9th Prince of Löwenstein-Wertheim-Rosenberg. He become Hereditary Prince in 1990 upon the death of his grandfather.