All we know is Arcann got her out of Nathama, granted we don't know enough, it was implied he may have rescued her, but now I'm thinking it was a simple case of Arcann was told to go and get her and she's just grateful someone took her away from that planet.
Valkorian: "Go get your sister!"
Arcann: "Okay."
Thexan: "Can I go too?"
Valkorian: "YOU'RE NOT IN THE BUDGET!"