Kousuke finds out that his older sister's boyfriend is unexpectedly sharp: it checks out because we know Rintaro is observant. He saw right through him! This kid's pretty precocious for looking out for his sister at his age. It's great that he's an understanding kid who realized what his sister was going through, and now I think he wasn't being as much of a brat as little brothers would've if it wasn't for their mother's illness.
I like how this chapter tells us even more about Kaoruko even when she wasn't walking with them. This girl was basically forced to be strong and grow up a little faster because of their mother's sickly constitution, and that's on top of her being the eldest child in an Asian family (bearer of parental expectations). It's not that she's "perfect" or "saintly", she just had to pick up some responsibility with matters around the house because their parents aren't always there or able to; She just can't afford to be a frivolous teen with Mrs. Waguri's frail health. Honestly, her situation kinda reminds me a bit of Sakuta from AoButa. Someone who was shaped by family circumstances to be quite the responsible older sibling.
This also likely puts into perspective Kaoruko's big appetite (especially for cakes) - maybe on top of cakes being her favorite food, she was also partly doing it to relieve stress. Someone like her who's incredibly busy would need some sort of outlet, and I can see her simple fondness for cake to be just that.