What to do.....
there aren't many women around with the quick, hale and beautiful perk. The next heir needs good stewardship values to hold all the holdings.
If they happen to be half-siblings without their parents being incestuous themselves, there is likely only a 10% chance of inbreeding. A risk I'm willing to take considering they are assassins and can assassinate the bad ones or throw the bad batch of babies in the dungeon.
https://ck3.paradoxwikis.com/Family_(relation)