What was the name of Cleopatra's first co-ruler and younger brother?

Cleopatra's first co-ruler and younger brother was Ptolemy XIII. When their father, Ptolemy XII, died in 51 BC, the approximately 18-year-old Cleopatra inherited the throne with her younger brother, who was about 11 years old.

Ptolemy XIII was not merely a political partner: Ptolemaic custom required royal siblings to marry and rule together. In practice, however, Cleopatra and Ptolemy XIII soon became rivals. His advisers, especially the eunuch Pothinus, helped push Cleopatra from power in 48 BC, forcing her to flee Egypt and raise an army.

Their dispute drew Julius Caesar into Egyptian politics. Caesar supported Cleopatra during the Alexandrian civil war, while Ptolemy XIII's forces fought against them. Ptolemy XIII died in 47 BC after the Battle of the Nile, reportedly drowning in the river while trying to escape. Cleopatra then ruled with their younger brother, Ptolemy XIV.

A common mix-up is Ptolemy XIV: he was Cleopatra's second brother and later co-ruler, not her first one.