Just my perspective but,

I think Ruby is actually living as Sarina, not as Ruby. From my POV it seems like Ruby's life is just like an extension of Sarina's life that got short life.

While it's different for Aqua. Right now he is living as Aqua, not the doctor he used to be. He changed, he got affected, he become a son and a brother. He left his old life, he is not the same person he used to be, regardless of whatever Ruby said about him being the same person. To me he isn't.

But Ruby, at this point, just thought that Ruby and Sarina is the same person. The fact that she can change her attitude so fast towards aqua after she knew that he is the doctor says everything. If she is living as Ruby, as HIS sister, she would be feeling the same uncomfort for kissing her brother, just like how she used to behave around her-used-to-be brother, Aqua (I mean, Aqua has always been overprotective of Ruby, but they, especially Ruby, never clings that much on Aqua before, just like normal brother sister relationship.

Now, in my eyes, she is just treating like Aqua and Ruby doesn't exist. Its just Sarina and her dear beloved doctor. Unlike Aqua that has a full realization that she is his sister, Ruby. Maybe because Aqua lived longer, entered adulthood and society, he is more mature, and is kind from the bottom of his heart, while Ruby is actually quite selfish because she is younger, she never experienced how it is to become an adult.

May be wrong but, I guess that would explain all this incest things she is doing. I'm not liking incest but, I'm thinking this way because I wanted to try to understand why the author made the flow like this. May be wrong tho lol