Zardnaar
Legend
The king can do whatever the heck he wants.
There might be political consequences, people chuffed about what she might or might not inherit, people who want their own children adopted, etc.
But it's his party, he can adopt who he wants, adopt who he wants, you would adopt too if kingship happened to you.
That's a more modern take. Kings could not do what they wanted. Some were even kinda weak where they essentially ruled by powerful Dukes consenting to it.