Opinion of father-in-law

[K-vald]

I get a slight modifier in "State Diplomacy". But there aren't any tooltips saying that you should get +10 for father-in-law.

 

[K-vald]

"+10 to opinion for aceppted marriage."

This is gained when you accept their marriage proposal. Lasts for five years I think.
It's why I always wait for them to send me an offer after betrothing two characters, rather than clicking the notifier right away.

Will try this since I only tried with offering marriage myself.

UPDATE: I can get it to work. Got a marriage request from the King of England and afterwards he had the +10 modifier for 'allowed a marriage'. Do you, OP, have a save game where this is not the case?