The problem was, Yumi has heard all of their talks.
However, it was in fact fights of intimacy, so Yumi could not protest a thing.

When the strong bonding was confirmed, Yumi could now do even things impossible when their emotions didn't quite reach each other. Rather, Yumi would like to try everything. Before, Yumi could only watch unbelievably at Yoshino and Rei-sama exchanging snaps. Now that Yumi realized that it felt good to speak straight about her feelings, and it did her body no good to swallow back her words, Yumi decided to do exactly that.
Even onee-sama's little retorts was enjoyable to listen to.

"Anyway,"
Sachiko-sama said as she touched her teacup with her finger and pulled her finger back.
"Oh, seconds? Just a moment."
Yumi stood up and hurried to the electric pot. Onee-sama had a couple of preferences about food, and cooled tea didn't suit her taste.

"Great that you know."
"Well..."
I am your petite soeur after all. Sachiko-sama returned a smile on her face, delighted that her feelings got through.

"See?" Yoshino said to Shimako. "Even packs of dogs won't eat bacteria like sisters' fights.*"
"I see. What a lesson!"
Hey, I heard everything! You other sisters, please stop teasing us.

"Amusement parks are crowded in the summer vacation, aren't they?"
Sipping the freshly prepared red tea in satisfaction, Sachiko-sama said.
"Yeah..."
Yumi gave a vague reply as she served the others with the warm tea. Every place for entertainment would be the same though.

"The School Festival is coming as well, you know."
"Argh"
Yumi was out of words as the unexpected School Festival excuse was pulled out.
What is that? What is that? Sachiko-sama is lining up small obstacles on the road towards the amusement park.

"Above all, I feel most uncomfortable by the heat."
There it went, the attacks of a princess' willfulness.
Pulling this excuse meant the end of conversation. Then not only a date at amusement park, any outings between the sisters during the summer vacation had been ruled out.

* Need revision on this line's translation:
「姉妹げんかは、犬どころかバクテリアだって食わないのよ」 <--A famous line of Yoshino as voted on the web, by the way.