Cleopatra married her brother

One of the most interesting facts about Cleopatra is that Cleopatra married her brother. Ptolemy XII died before March 22, 51 BC, which was the date of Cleopatra's first recorded act as queen: her journey to Hermonthis, near Thebes, to install a new sacred Buchis bull (worshiped as an intermediary for the god Montu in ancient Egyptian religion). The Roman Senate was not informed of Ptolemy's death until 30 June or 1 August 51 BC; Cleopatra may have concealed the news until she could win the crown. Cleopatra most likely married her brother, Ptolemy XIII, but the relationship is unknown. Ptolemy II and his sister, Arsinoe II, initiated the incestuous Ptolemaic practice of sibling marriage, but the long-held royal Egyptian ritual was regarded obscene by contemporary Greeks.

Although prominent opponents of this incestuous practice were occasionally ruthlessly punished and suppressed under the combined reign of Ptolemy II and Arsinoe II, by the time of Cleopatra's reign, sibling marriages were regarded as a regular arrangement for Ptolemaic kings. By 29 August 51 BC, official records began listing Cleopatra as the sole ruler, indicating that she had rejected her brother as a co-ruler.

Despite Cleopatra's rejection, Ptolemy XIII retained powerful allies, most notably his childhood tutor, regent, and administrator of his domains, eunuch Potheinos. Achilles, a notable military leader, and Theodotus of Chios, another tutor of Ptolemy XIII, were also members of the plot against Cleopatra. Cleopatra appears to have formed a brief alliance with her brother Ptolemy XIV, but by the autumn of 50 BC, Ptolemy XIII had gained the upper hand in their battle and began signing documents with his name first, followed by the formation of his first regnal date in 49 BC.