Complex sentence: Her collection included Bach's organ sonatas, which eventually were passed on to her grandnephew Felix Mendelssohn; there was also a copy of the two harpsichord arrangement of the organ sonatas in the Itzig household, belonging to Sara's sister Fanny.
Split into simpler sentences: Her collection included Bach's organ sonatas, which eventually were passed on to her grandnephew Felix Mendelssohn. There was also a copy of the two harsichord arrangement of the organ sonatas in the Itzig household, belonging to Sara's sister Fanny.