Why does the woman that Dr. Manette writes of count to twelve over and over as part of her feverish ravings?A Tale of Two Cities by Charles Dickens

In Book the Third, Chapter X of A Tale of Two
Cities, the letter of Dr. Manette is produced in the trial of Charles Evremonde,
called Darnay. This letter is offered as evidence that Dr. Manette has denounced the family of
Evremonde. From the reading of the letter, those listening learn that Manette was taken by one of
the Evremonde twins and brought to a house by an overrun fountain. There a young, beautiful woman
lay with her arms bound to her sides with sashes and handerchiefs. When Dr. Manette looks at her,
he notices that her eyes are dilated and wild; shrieks come from her constantly, and she repeats,
"My husband, my father, and my brother!" Then, she counts to twelve and says, "Hush!" Over and
over she repeats these words that Dr. Manette comes to realize mean the hour of twelve
o'clock.

Her brother, who is dying of a sword thrust tells Dr.
Manette that his sister was recently married. At their home, the husband was ill, and she tended
him. However, the one Evremonde brother admired her beauty and asked the husband to "lend her to
him." To persuade the husband to make his sister willing, they put him in a harness throughout
the day, and kept him in the "unwholesome mists at night." The husband became deathly ill
and

he sobbed twelve
times, once for every stroke of the bell, and died on her
bosom.

When the brother saw the sister
being taken away by the Evremondes, he rushed to tell his father, whose heart "burst" upon
hearing this terrible news, and he never spoke another word, having suffered a stroke. Thus, the
young woman's words indicate her grief over the damages to her wounded brother and her stricken
father, and the death of her husband at the twelfth hour.