There's nothing that I really disagree with on the emotional level, so you can just take that and disregard whatever I'm going to say right now. I mean, I've been pretty vocal against Chika's current arc and on how much I love Yumi and her development through multiple posts...
That being said, Chika's reaction is perfectly understandable and logical imo (as it concerns Yumi's event). Her own event is where I think Selebus drops the ball.
So hey, here comes Chika's greatest critic to her defense this time: Chika has told Yumi a hundred times that it was obvious she liked Akira. She has tried having an honest talk with her, she has tried cucking her, she might have even tried inviting her to a threesome. Most important of all, Chika has told Yumi no cap that if Yumi didn't come clear with her feelings, it was bound to blow up in their faces and it would ruin their relationship.
That's what happened. What Chika saw, even though on the surface was a misunderstanding, at the core it very much wasn't and she says so this during this last update. Yes, Akira wasn't cheating on her with Yumi, and neither they went back to their bedroom to have sex. She was wrong about that.
But what she was not wrong about is that they both were lost in eachother's eyes in a manner that just wasn't ever true with her. That Yumi was not coerced into that situation and that she wanted to kiss him back. That she has for a long time. That she let this blow up in Chika's face when she could have very easily just said so for years now.
Now, is part of this Chika's fault for not listening to the so many people that tried to open her eyes (Akira included)? Yes, 100%. But her reaction is still valid, because if we look to another example of surrogate sisters, Miku didn't wait a single day to tell Makoto she kissed Akira and to apologise.
This is guesswork, but if Yumi were to come clean with Chika before she confronted her, I think this would have gone down very differently. She didn't, in a huge part because Chika is insane. And in a smaller one because nothing really happened from her perspective.
But, and that is the point of this post, Chika has every right to lose her cool with Yumi. I wouldn't blame her at all on that front to be honest, she'll probably apologise for the things she got wrong, and when she does, the things she got right will more than compensate for the emotional distress of the rest. Yumi has fucked up, she has feelings for him and she let herself be in that situation with her sister's boyfriend.
If Chika didn't see it, it would have been fine. But now that she saw it, there's a whole arc of things she didn't see that become evident to her. They didn't get to that point from zero, and it's all been going on behind her back, and even before they were a thing, with Yumi keeping all that a secret from her (while she asked her about it many times over).