Yotsuyu specifically mentions forcing her to marry that dignitary was HIS idea, and that she had to grin and bare it partly for his sake.

I'm leaning more towards this dignitary giving Asahi better opportunities within Garlemald. Endearing his clan to this powerful figure could be quickly done through a political marriage, and he had a sister right there.

She was basically a piece of property to his family, anyway, so why not sell her to make friends in high places?