Oddly enough, she is a Countess. The Anglo-Saxon word Jarl was used in England to replace the Latin Comes - Comte in French, Count in English. The Comes was the head of a Comitatis - modern County. Under the Norman Conquest, the Earl became a local ruler, but his wife was described in Norman French, hence Countess.
In the British nobility, a viscount is ranked immediately above a baron and immediately below an earl. I doubt whether a viscount would "serve under an earl" as you put it.
earl
The French Vampire, Viscount de Morieve, lived during the French Revolution, if legend is correct, and for nearly a hundred years after that, before his grandson had him staked.
Viscount is a title held by certain British noblemen, and ranks below an earl and above a baron. The first person to be titled a viscount was John Beaumont, who received the honor in 1440. Today the title is usually given to men whome the ruler wishes to honor.
duchess
The gender noun of "viscount" is "viscountess." While "viscount" refers to a male holder of the title, "viscountess" refers to a female holder of the title, typically the wife of a viscount or a woman who holds the title in her own right.
A vicomtesse - note the spelling - is the wife or widow of a vicomte. That's the French versions. In English we talk of a viscount (the male) and viscountess, h is wife or widow.
The wife or widow of an earl or count; a member of the British peerage ranking below a marquess and above a viscount
The wife or widow of an earl or count; a member of the British peerage ranking below a marquess and above a viscount
Marcy Stewart has written: 'Charity's gambit' 'The Viscount Takes A Wife' 'The bridegroom and the baby'
Viscount Exmouth was created in 1816.
Viscount Palmerston was created in 1723.
Viscount Bangor was created in 1781.
Viscount Scarsdale was created in 1636.
Viscount Sidmouth was created in 1805.
Viscount Doneraile was created in 1703.
Viscount Hawarden was created in 1705.