Responses
Uh...no? Because the sibling-like relationship has existed between the Duchy and the Royal Family for over a decade (since the Duke and Duchess were alive), and has nothing to do with Hari. It's not necessary for the ties between the two families to get closer.
If it WAS necessary for Hari to go under the protection of a different family (which it is not), Johan's family actually raised her for several years and already considers her family.
But it isn't necessary. So. ╮(￣▽￣)╭
I think what could happen is Hari is disowned by the ducal family or whatever and adopted by the Royal family for the sake of marrying Eugene. I think that's why they specifically established a sibling like relationship with the crown prince?