Miyuki is an obvious source of contention for both Haru and the reader, but fortunately it seems that Haru has progressed & strengthened enough that she didn't see the sister's insults and threats break her resolve, or even really make her stumble.
She couldn't say anything back in the moment, but that's not necessarily a bad thing. I wish that she would tell Yuu, and not keep it a secret. I understand why she didn't in this chapter, as Yuu's currently feverish and it's more important narratively to continue establishing Yuu's shifting feelings for Haru (even if she's currently delirious, and can "hide" what happened behind her being sick for the moment).
But Yuu does need to know. Because sadly, she seems very hung up on wanting her mother's approval still, rather than having any significant semblance of anger or resentment for being abandoned and then never contacted.
Miyuki returning out of nowhere and threatening & more importantly demeaning Haru like this, might change the calculus for Yuu and make her less prone to simply falling into the desire to see her sister again, which would put a ton of strain on Haru to witness, especially after this first contact.
They need to be united and in sync more than ever; so full honesty between them is key, and that includes being aware of Miyuki's behavior.
But, thanks for the TL.