@Dradetias
She isn't interested ONLY in Kei. She's interested in the
relationship Yokoe and Kei have because it is really weird how close they are.
The misunderstanding started a few chapters back when Yatsude asked Kei how she was going to pass her vacation and the latter answered that she would pass it all with Yokoe. Which takes us to a chapter where Yatsude goes to Yokoe's house specifically to investigate if the both of them were a couple, but as stated in this chapter she concluded Yokoe's acting way too gross to be in a relationship in the first place.
And then Tsuu-chan completely destroys her reasoning with facts and logic throwing Yatsude for a giant loop just at the end of this chapter.
And which character could be best to understand Yokoe's struggles, when she's struggling herself with her sister complex?