Aerope

Aerope is the daughter of Catreus, a king of Crete.  He sold her into slavery.  In Mycenae, Atreus purchased her as a wife for himself.  She bore him two sons, Agamemnon and Menelaus.  She did not love Atreus, however, and had a secret affair with Thyestes.  While together, she told Thyestes of Atreus' secret treasure: a golden fleece from a sheep he was supposed to have sacrificed to Athena.

When Sthenelus' son Eurystheus died, Atreus and Thyestes were the two principal contenders for the throne.  Thyestes proposed that the throne should be given to the man, who could produce a golden fleece.  While trying to conceal his delight over such incredible luck, Atreus agreed.  What he did not know, however, was that his wife Aerope had already stolen the fleece and given it to Thyestes.  When Thyestes produced the fleece, he was given the kingdom.

Source(s):

  1. ApollodorusBibliotece.