Daughter of Ptolemy I Soter and queen of Thrace (300-281) and of Egypt (277-270). She married the king of Thrace (300) and tried to have her son made heir instead of Agathocles, the king's son by an earlier marriage. Agathocles sought help from Seleucids, causing a war in which Arsinoe's husband was killed in battle. Her half brother, who took power in Thrace and Macedonia, cajoled her into marrying him and then promptly killed her two younger sons. She fled to Alexandria, ousted the wife of her brother Ptolemy II, and married him (c. 277); like the pharaohs, the two were called “Philadelphoi” (“Sibling-loving”). She wielded great power and shared many honors with Ptolemy, including deification while alive.