Complex sentence: As tsarevich, and then as tsar, Alexander had an extremely poor relationship with his brother, Grand Duke Vladimir, a rivalry that was shared between Maria Feodorovna and Vladimir's wife, Grand Duchess Marie Pavlovna.
Split into simpler sentences: As tsarevich, and then as tsar, Alexander had an extremely poor relationship with his brother Grand Duke Vladimir. This tension was reflected in rivalry between Maria Feodorovna and Vladimir's wife, Grand Duchess Marie Pavlovna.