The daughter of Antiochus III became queen of Egypt as part of a treaty between her father & Ptolemy V Epiphanes (193 BCE). After her husband's death she became co-regent of Egypt with her son, Ptolemy VI.  Despite the fact that she was a Seleucid, she kept Egypt neutral in the conflicts between Rome & her brothers, Seleucus IV & Antiochus IV.

References: Josephus, Antiquities 12.154, 167, 185, 204, 217.

Other resources on line:

• Cleopatra I - detailed notes in Chris Bennett's Egyptian Royal Genealogy.

• Cleopatra I Syra - Jona Lendering outlines key events [Livius: Articles in Ancient History].