St Scholastica (c480-543) was the twin sister of St Benedict. She originally set up a monastery at Subiaco, and followed her brother when he moved to Monte Cassino, at the nearby monastery of Plumbariola.
Most of the information we have about her comes from St Gregory's Dialogues, including the story of their famous last day together, depicted in the painting above. You can read the relevant parts of St Gregory on her here. St Gregory depicts her as outdoing her brother in holiness, and providing a charismatic counter to his insistence on following the rules to the letter!
Her feast is celebrated as a Class I by Benedictine nuns, in part because, as well as attesting to the tradition of twinned monasteries, she also lends support for the fact that originally at least, Benedictine nuns were not established as a "second Order", with stricter enclosure requirements than for men as for later Orders, but rather the provisions of the Rule generally apply equally to both monks and nuns. Indeed, there is a nice letter (which may be a later medieval pious fraud, but nonetheless attests to the point) attributed to St Scholastica on this very point, which you can read over at Vultus Christi.
A Song about St Scholastica
Finally, from the Monastic matrix project, courtesy of Logismoi, a song by St Aldhelm, Bishop of Sherborne (c 639-709):
Scholastica took her very name from schola,
God enriches her abundantly with heavenly favour,
She who gained golden rewards by her virginal vow.
Concerning whom a little twig of nourishing life
is wont to scatter excellence
as widely as the world extends.
Because the virgin impatiently urges her brother
who is joined to her by a covenant of kinship,
and supports her pleas with reasoned argument
So that, at night, they might partake
of the sweet courses of the holy books
and the banquets of the holy word.
From which the breasts of many
are sufficiently filled,
And the hearts of holy people nourished.
But the faithful brother is not moved by any pleas,
Nay rather he disdains his holy sister in his words.
Then the virgin urged the good Christ in her heart
to deign to heal the wound of sorrow for her.
Thus soon the whole sky grows dark
with a stormy whirlwind
and the vault of the heaven with gloomy air.
Huge rumbling thunder,
mingled with flashing lightning bolts,
And the Earth quaked,
trembling from the great noise.
Wet fleecy clouds moisten it with dewy drops,
And the air bedews the land with gloomy showers.
The valleys are filled
and abundant streams overflow,
Then unwillingly he remained
who before had deliberately refused
what his distressed and weeping sister had sought.
So God heeds those who ask with burning heart,
Even when they pay attention to words
which do not console.
(translated by Mary Forman, OSB, and originally published in Vox Benedictiana, 1990)