i'm talking impossibles, so rather than remove ribs i would just imagine she was born with a slightly different figure that would allow her a proportionally slimmer waist, naturally. kinda like KonKon, who has a similar build (as opposed to someone like Aichan). even if KonKon was as flat as Reina up top, her waist-to-hip ratio keeps her curves.
though of course, if i'm doing this i could just say "if she were born with an entirely different body that's perfect".

to clarify for people like resop, i wouldn't want Reina to actually do anything to her body. just making observations on her look, and imagining what she might look like with slight alterations. i think she's fine the way she is though.