Complex sentence: She lived most of her remarkably long life in Calcutta, which in 1772 became the "de facto" capital of British India, and so witnessed a great deal of the history of Kolkata.
Split into simpler sentences: She lived most of her remarkably long life in Calcutta, which in 1772 became the "de facto" capital of British India. She witnessed a great deal of the history of Kolkata.