Complex sentence: Because Rudolf was the only son of Franz Joseph, Emperor Franz Joseph's brother, Karl Ludwig, became heir-presumptive to the Austro-Hungarian Empire, but he renounced his succession rights a few days later in favour of his eldest son Franz Ferdinand.
Split into simpler sentences: Because Rudolf was the only son of Franz Joseph, the emperor's brother, Karl Ludwig, became heir-presumptive to the Austro-Hungarian Empire. However, he renounced his succession rights a few days later in favour of his eldest son, Franz Ferdinand.