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 Hawarden was created in 1705.
Viscount of Oxfuird was created in 1627.
Viscount Ridley was created in 1756.
Viscount Torrington was created in 1721.
Viscount Valentia was created in 1620.
Viscount Hereford was created in 1611.
Viscount Exmouth was created in 1816.