A few details will probably make you say, "Oooh, I see now!"
Makoto explains that when he heard about Kanami’s pain concerning her brother, and her loneliness, he couldn’t stand by and do nothing about it. That’s when he thought of the rental idea. He hoped the rental periods would help ease her emotional wounds. He wanted to become her strength. This leads to Kanami asking him his name.
Kanami thinks about all the things she wants to learn about Makoto-oniichan, about who he is. The wording for this is 「慎おにいちゃんのこと」. When referring to a person, こと sort of encompasses everything about a person, their essence. It’s the way the walk, the way they talk, how they interact with others, how they spend their time, why they do what they do. Wanting to know all about his こと means Kanami wants to know all about Makoto.
Then, in her mind, Kanami imagines her real brother asking, 「俺のことはどうでもいいの？」
(I’m entering grammar mode now.)
The prior topic was about Makoto’s こと, so this line shifts the topic to Kazutaka’s こと (「俺のことは」). Then, a comment is made on this new topic: 「どうでもいいの？」
Putting this comment into a Japanese-to-English dictionary gives you:
- [exp,adj-ix] inconsequential; indifferent; not worth worrying about; trivial
- [exp] whatever; whichever; I don’t care
It’s possible to break this expression down into the sum of its parts, but I’ve never tried, so I’ll take this translation as a whole.
Her brother’s imagined words in Kanami’s mind ask, “So you don’t care about my こと?” Essentially, it comes across as, “You want to get to know Makoto so much that you don’t care about me anymore?”
Notice that this isn’t her mean big brother making a spiteful comment. He’s smiling. It’s her kind big brother. If Kanami gives all her focus and attention to Makoto, does that mean she’s no longer thinking of getting her kind brother back?
Kanami realizes she can’t let that happen. 「だめだ」 She knows she pays money for the rental system to keep from seeing Makoto as her real brother. And if that happens, she’ll never get her real brother back.
Makoto suggests ending the rental system. Kanami can just call him normally.
Kanami says it’s all right, and she wants to continue the rental system. She thinks to herself that she needs to properly use the rental system, and stop her feelings (of wanting to get to know all about Makoto). She doesn’t want her thoughts of Makoto to go beyond those for her real brother.