Hime Kishi wa Barbaroi no Yome - Vol. 3 Ch. 11 - The agreement is the figure of the neck ring.

[MangaDex]

 

[SteamBunky]

I can feel "home sweet Alabama" coming inside me

 

[dude-e]

They’re so adorkable sometimes

 

[Derksy]

Nothing better than marrying your young and cute looking Aunt.

 

[DisSiteSux]

Surprise incest.

 

[GrumpyGizmo]

Goated chapter. Okay first of all, incest was very common irl monarchies so it makes sense they wouldn't give a damn about it, specially in these lands with their own traditions (the story of how they are pretty much descendants from the kingdom was a nice addition to the lore, but we could suspect since he told about that way to get a wife for warriors in the skirmishes against the expeditionary trops). Second, I'm happy she finally met her long lost cousin (yeah let's go with that. She wasn't her sister) and last but not least that love confession was very cool and romantic.