Some families with deep karmic ties and a fierce, ruthless nature especially like to find simple, bright, kind, and virtuous girls to be their daughters-in-law.

They use the virtue of the other family to dilute their own karmic debt, so that their descendants can be a little better.
Generally, these chosen girls are also very innocent, and their parents are kind and generous people.
So, it's basically a lamb walking into a tiger's den, a little deer jumping into a python's lair.