-Kei likes Himari
-Kei can't admit she likes her or confess.
-The fact that Himari might like Haruki is a threat to her.
-Kei knows Himari is even more naive and immature and gets mixed signals.
-Everyone in this story has no love relationship experience outside of media and manga. (because parents don't exist).
The issue with Kei is that the author used her for a stupid sub-plot that literally lead nowhere. It didn't resolve Kei's feelings properly as her whole conflict was the fact that she didn't want to push her feelings onto Himari who didn't understand love. I can't even tell if Kei still likes Himari in that way or not. Does she really want her to be happy or is she just happy because haruki isn't viewed like that from his sister. That and making her a Tsundere (if you can call it that) to two different people is kind of trash. Of course people are going to dislike someone who hits Haruki and coos over his sister.
Maybe if it was played up that Kei was envious that Haruki had a sibling (a "cute" one like Himari). It gives explanation to why Kei was so pushy as a kid and didn't understand boundaries. Or if you're going to use the yuri drama. To actually lean into it more than to half ass it give flashbacks, background information. Something that would let the reader know she's full yuri or even understands that. Kei comes off as someone who is aggressive against everyone that isn't a girl for shallow reasons. This chapter made her show humility and reflect on her actions but people are only going to view it as something she only did because Himari didn't love Haruki as a man.
Haruki has been a only child for a long time and the closest thing he had to a sibling was pushy and overbearing. Whether they admit it or not, Kei and Haruki being close enough to be in each other's room and not get the cliche "I'm in a girl's/guy's room" is proof enough. He views everyone as too high energy, especially Himari. He just feels more responsible for her and can't hide in his room since Himari is a danger to herself.
If Himari is really the Titular character, she should be out here changing lives like she should. I would have started with the brother more since Kei wasn't characterized as a main character. and our knowledge of her background ends at "Childhood friend, nextdoor neighbor". It's obvious she is now seeing how she got a whole ass arc for herself right off the jump.