Complex sentence: Elena was a close friend of her brother-in-law, Alexander I of Russia and his wife the Empress Elizabeth Alexeievna and she was also quick to befriend the shy Maria Alexandrovna, the wife of the then Tsarevich Alexander.
Split into simpler sentences: Elena was a close friend of her brother-in-law, Alexander I of Russia and his wife the Empress Elizabeth Alexeievna. She was also quick to befriend the shy Maria Alexandrovna, the wife of the then Tsarevich Alexander.