Xav Vorbarra (vohr-BAA-ruh) was a Prince of Barrayar and a cadet member of House Vorbarra. He was the son of Dorca Vorbarra, the half-brother of Yuri Vorbarra, as well as the brother of the wife of Ezar Vorbarra. In his youth, he served as ambassador to Beta Colony, and married a Betan woman. During the First Cetagandan War, he smuggled galactic weapons to the resistance.
He had three children: the eldest was Olivia Vorbarra Vorkosigan, then an unnamed son, and youngest was Sonia Vorbarra Vorpatril. His son and elder daughter and several grandchildren were killed in Yuri's Massacre, after which he and Piotr Vorkosigan began Yuri Vorbarra's Civil War to overthrow Emperor Yuri. He and his wife sometimes had their grandson Aral to stay after the war ended. 
Behind the scenes
- Yuri was born first, to Dorca's first wife/empress, who by the way was a Vorrutyer of some high rank/standing. Dorca later took up a long-running affair with some (unnamed, sorry) other Vor lady of high blood -- such high-born openly held concubines are not unusual in royal histories. It is unknown if she was married to anyone else as well, though this is not ruled out. (Royal mistresses in some Old Earth cases were sometimes socially _required_ to be married.)
- In due course, Empress #1 passed away of natural causes. After a sufficient interval, Dorca was then free to wed his long-standing Vor concubine (who must have been quite something), and legitimated any and all children they had by then. An adult or near-adult Xav might well have attended his parents' wedding.