Responses
I don't think it was a smirk, she just acknoweldged the fact that she was "as dirty" as her brother for loving him ; her face looks defeated/shamefull bc she is letting those "nasty" feelings take the lead and she knows how wrong it is
That was not a smirk, nor was she gloating or doing any kind of plan. That was a smirk of acceptance that she had given in to her desires and accepted her love despite it being wrong.
I think it was page 78 (when Himeko & her brother came back from the park) you can see her smirking, like she just watched the last piece of her plan fall into place, and Tsubaki saw that which was why he ran into his room saying "why can't that woman just die?!"
The sisters in these stories are not helpless lambs. Even the 2nd sister started taking the lead after she recalled their past promise.