Perhaps in korean he's calling her noona, which can be used to by guys to refer to their older sisters, or to refer to older girls that they are close to. So they can use that...
For the first part: I’m sure she’s going to make up how her brother was outcasted and perhaps her family adopted him so they were raised as brother and sister. For the next part: she based the spoiled brat prince after her stuck up little brother who she was always compared to growing up. She didn’t know Barron would become a little psychotic shit because she never wrote for that to happen. Making Barron look like her brother was supposed to be a subtle fuck you to the brother.
Have you ever heard of the iron mask?
But that would still mean he's close to her ._.
Even if she does explain that they are twins he still calls her sister so that still means that she's related to the current emperor unless she plans to explain that in like the next few chapters I guess? Also I love how even her brother recognized how handsome Edward is LMAO but her brother seems like a nice guy why would she make him the image of the evil emperor? Makes me sad