How did Cleopatra VII first become co-ruler of Egypt with Ptolemy XIII?

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Ptolemaic sculpture — Ptolemaic dynasty Head of a queen, marble sculpture possibly made in Egypt, Ptolemaic period (305–30 bce); in the Brooklyn Museum, New York. 14 × 12 × 11.5 cm.

Cleopatra VII became co-ruler of Egypt with her brother, Ptolemy XIII, because their father, Ptolemy XII Auletes, proclaimed them as co-regents shortly before his death in 51 BCE.123

Following their father's passing, the throne passed to the two siblings, and it is likely, though not definitively proven, that they married soon after to solidify their joint rule. Although they were co-rulers, the 18-year-old Cleopatra was approximately eight years older than her brother and quickly established herself as the dominant partner in the administration. Evidence of this power dynamic is noted in historical records, which show that it was not until October of 50 BCE that Ptolemy XIII’s name began to appear before Cleopatra’s in official decrees.2