I think reading into it that Rebecca joins the crew is weird, especially since Luffy is upset that she and Kyros can't be together. He clearly wants to unite the two. Having her join the crew would separate them again, thus spoiling the whole emotional centerpoint of this arc.
As I said before, Luffy might make a declaration to the marines that she's in cahoots with him, and that will mean she'll have to go into hiding as she's a criminal and would no longer be able to associate with the Royal Family, exactly like what her father did.
The whole theme of the entire arc is about maintaining false appearances to save face. Doflamingo turned Dressrosa into a land where people literally turned a blind eye to the problems in the underbelly until luffy's crew exposed everything. The Marines faced the same issue where they had covered up everything regarding the Sichibukai in order to save face, but even this was exposed by Issho. The Royal Family had to make up Scarlet's death in order to allow her and Kyros to be together, and now something very similar will happen with Rebbeca for the same thing to happen.
Oda is pretty symmetrical with his internal themes, and I see no exceptions here.