In truth, Elwing had once offered this to Ri. However, context is very important.
Firstly, Elwing had offered it as a betrothal gift. If Ri became a member of the Belmonts, it obviously wouldn't matter if she was given a portion of their bloodline. Secondly, think of how close the Belmonts are with the Acacias. Such a betrothal was something even King Acacia had wanted.
In the end though, all of their plans of a sneak attack were thrown out of the window completely by Meiying. Had Meiying not taken the box, it might have still worked, because the Daiyu could still pretend to fail since Meiying had no idea about the connection between the Ragnors and Daiyu. However, because she knew the feathers existed, the entire plan was ruined!
This forced Patriarch Ragnor to attack early, getting the leg up on the Belmonts before they were ready.
