king cannot have daughters
young princess is adopted
older princess is the new queen's daughter

single king, no sons, no natural heirs, no proper queen
king married in an attempt to save face.  queen has a biological daughter... didn't work
bastard princesses are scandalous,  no kingdoms are interested
promised his subjects he'd make the kingdom powerful  (trying to regain their favor)
getting desparate for any powerful alliance

younger princess is gullible