Complex sentence: Her husband was the 2nd Viscount Melbourne, the Prime Minister, however, she was never the "Viscountess Melbourne" because she died before he succeeded to the peerage; hence, she is known to history as "Lady" Caroline Lamb.
Split into simpler sentences: Her husband was the 2nd Viscount Melbourne, the Prime Minister. However, she was never the "Viscountess Melbourne" because she died before Melbourne succeeded to the peerage; hence, she is known to history as "Lady" Caroline Lamb.