Given the question: Question: "What happened when there was a forced coupling of children?"  Context: "The Greek kingdom of Bactria began as a breakaway satrapy of the Seleucid empire, which, because of the size of the empire, had significant freedom from central control. Between 255-246 BCE, the governor of Bactria, Sogdiana and Margiana (most of present-day Afghanistan), one Diodotus, took this process to its logical extreme and declared himself king. Diodotus II, son of Diodotus, was overthrown in about 230 BC by Euthydemus, possibly the satrap of Sogdiana, who then started his own dynasty. In c. 210 BC, the Greco-Bactrian kingdom was invaded by a resurgent Seleucid empire under Antiochus III. While victorious in the field, it seems Antiochus came to realise that there were advantages in the status quo (perhaps sensing that Bactria could not be governed from Syria), and married one of his daughters to Euthydemus's son, thus legitimising the Greco-Bactria dynasty. Soon afterwards the Greco-Bactrian kingdom seems to have expanded, possibly taking advantage of the defeat of the Parthian king Arsaces II by Antiochus."  Answer:
The answer is:
legitimising the Greco-Bactria dynasty