She had vowed that her brother would avenge her.

It only names Yrsa and the situation that her son and brother (i.e. Hroðulf) will avenge Fródi (Froda):

After some adventures, the two brothers burn their uncle to death inside his house and avenge their father.

"Lily Clegg's brother, come to avenge her!"

Al-Khansa' insisted that her brother, Sakhr, avenge Muʿawiyah's death, which he did.

His younger brother, Hardheri (replaced in later German legend by Wolfdietrich), avenged Ortnit by killing the dragon, and then married his brother's widow.

A few weeks later, as Muhammad 'Ali tells it, he and his brothers avenged their father's death by murdering Ahmed Isma'il.

For now comes his brother, Simon Materna, avenges Gregor Materna, and regardless of the seasons sets fire to timber-frame houses and proud-gabled granaries.

When she dies from grief, the younger brother avenges her death by killing his sibling.

"His brothers will avenge him," he said.