Complex sentence: In 1855, when she was 19, she had a baby girl, Alice Hensler, from unknown father, but, for sure, it was a member of the Milanese nobility, of whom she was a lover.
Split into simpler sentences: In 1855, when she was nineteen, she had a daughter, Alice Hensler, from an unknown father. The father was certainly a member of the Milanese nobility, of whom she was a lover.