[coin depicting Arsinoë, commissioned by her son Ptolemy III, 3rd century BCE, Kestner Museum, Hanover, source: Wikimedia Commons]
Arsinoë was the daughter of Lysimachos (formerly one of Alexander the Great’s bodyguards) and one of his wives, probably either Nicaea or Amastris.* It’s possible, even likely, that she knew Arsinoë II during her youth, considering her future sister-in-law married Lysimachos as his third wife when Arsinoë was a young child.
She married Ptolemy II sometime between 285 and 281 BCE, after he began co-ruling with his father but before her own father’s death. The couple had three children together, Ptolemy III, Lysimachos, and Berenike Phernophoros. This, however, would not save her from repudiation and exile. Her father’s death in 281 left her vulnerable, though not completely without allies.
At some point, Ptolemy II accused Arsinoë of plotting against him and had her exiled to Coptos, a place still within Egypt. Precisely when this happened or whether she actually was planning to move against her husband is uncertain. Certainly it was after her father’s death deprived her of her worth as part of a marriage alliance, though how long afterwards is impossible to say. Many automatically assume this happened before Ptolemy’s marriage to his sister, Arsinoë II, but there isn’t really any direct evidence either way. We also have no way of knowing if Arsinoë II had anything to do with Arsinoë I’s fall from power. It’s possible, certainly, but it’s also possible she wasn’t involved at all.
What we do know is this: at some point Arsinoë I was accused of treason. Two of her prominent supporters were executed and she herself was sentenced to exile within Egypt. She was not sent out of the country or killed and her children were legally adopted as the children of Arsinoë II, though whether this was before or after Arsinoë II’s death is unknown. As for the woman herself, the historical record says no more on Arsinoë I.
*It’s technically possible but unlikely that Arsinoë II could have been her mother.