St. Francis of Assisi famously inspired St. Clare to lead a similar life of poverty and prayer, eventually forming a separate religious order that would be called the Poor Clares.
St. Clare wasn't alone in her desire to lead a life of radical holiness, as her younger sister followed her into the convent.
This enraged their father, who did not want his daughters to lead a life of seclusion in a monastery. He sent their brother, Monaldo, with several armed men to drag Agnes out of the convent by force if necessary.
However, something miraculous happened, as the Catholic Encyclopedia explains.
Monaldo, beside himself with rage, drew his sword to strike the young girl, but his arm dropped, withered and useless, by his side; others dragged Agnes out of the monastery by the hair, striking her, and even kicking her repeatedly. Presently St. Clare came to the rescue, and of a sudden Agnes's body became so heavy that the soldiers having tried in vain to carry her off, dropped her, half dead, in a field near the monastery. Overcome by a spiritual power against which physical force availed not, Agnes's relatives were obliged to withdraw and to allow her to remain with St. Clare.
St. Agnes was eventually chosen by St. Francis to be the superior of a community of sisters in Florence, Italy.
She died on November 16, 1253, and was buried next to her older sister, St. Clare.