Syphilis? Could be. Also cholera, typhoid, tuberculosis, diabetes, childbirth. Any number of things. We will never know what she died of, esp. since she was a fictional character.

The painter Mary Cassett had a sister who died young (in her twenties or thirties) of kidney disease. Her family had money and access to doctors, but no one could save her. Such was medicine in the 19th C.