Complex sentence: It was the first museum in France to open its collections to modern artists, at the beginning of the 20th century, and its collection of modern and contemporary art has grown to become one of the largest in Europe.
Split into simpler sentences: It was the first museum in France that opened its collections to modern artists, at the beginning of the 20th century. Since then its collection of modern and contemporary art has grown to become one of the largest in Europe.