According to the Roman historian Plutarch, Julius Caesar had two sisters, both of whom were named Julia. The elder, Julia Caesaris Major, is known only through a reference to her grandsons, while the younger, Julia Caesaris Minor, played a larger role in her brother's life.
Julia was close with her family, and Wikipedia explains that she gave the legal courts insight into her brother’s life and marriage to Pompeia. Caesar's second wife, Pompeia, allegedly had an affair with Publius Clodius Pulcher, a Roman politician. Caesar's younger sister and his mother, Aurelia, went to court as to give testimony proving Pompeia's scandalous behavior. She married a prominent Roman politician, Marcus Atius Balbus, and she was the grandmother of Gaius Octavius, who later became Augustus, the first emperor of Rome. These facts are revealed in text from "Lives of the Twelve Caesars" by Suetonius, reprinted on a University of Chicago web page.