Complex sentence: She is possibly best remembered for writing the song "I Love You Truly", for which she became the first woman to sell one million copies of a song; that parlor song continues to be used at weddings over a century after its release.
Split into simpler sentences: She is possibly best remembered for writing the song "I Love You Truly", for which she became the first woman to sell one million copies of a song. That parlor song continues to be used at weddings over a century after its release.