Complex sentence: Her second obstacle in being happily in love with Pedro is that he agrees to marry Rosaura because he loves Tita so much that he is willing to marry her sister in order to be closer to her.
Split into simpler sentences: Her second obstacle in being happily in love with Pedro is that he agrees to marry Rosaura. Pedro's reasoning for doing this is because he loves Tita so much that he was willing to marry her sister in order to be closer to her.