Through her mother, Agnes was a descendant of the Salian dynasty, which ruled the Holy Roman Empire since 1024 until her maternal uncle, Emperor Henry V, died without issue in 1125. She was the half-sister of Duke Frederick II of Swabia and King Conrad III of Germany, both born from her mother's first marriage with Duke Frederick I of Swabia. Of her full siblings, one sister, Judith, married Marquess William V of Montferrat, and one brother was Bishop Otto of Freising, a renowned medieval chronicler.