Responses
It's not adoption but rather sponsoring and eventualy making them the daughter in law. So from the beginning, she was meant to be with Asiel. But you are right, the meaning is to add new blood in the aristocracy and to secure a powerful individual ( even thought they didn't know she will so powerful) instead of leaving them in the nature and they could Come to challenge the aristocracy hegemony
While the age gap is 5 years (not as crazy but still crazy), the way the characters are drawn, it makes them look older. Sienna is 12 when she was adopted and Asniel was 16-17 years old but he is drawn as if he's 25. It does make me uncomfortable that he's the ml rather than Micheal or heck even Lou. The whole adoption thing is weird cause they aren't adopting for the sake of having another child but rather having a powerful teraformer and a name bearer for the family to claim the power as theirs.