Complex sentence: The opus was dedicated to Anatoly Tchaikovsky, one of the composer's two brothers, who had helped Tchaikovsky through a difficult emotional period in 1877 when the composer married and separated from Antonina Miliukova.
Split into simpler sentences: The opus was dedicated to Anatoly Tchaikovsky, one of the composer's two brothers, who had helped Tchaikovsky through a difficult emotional period in 1877. It was during that year when the composer married and separated from Antonina Miliukova.