Regarding the passive form of verbs in Japanese, there’s something I wish I knew about it a long time ago, as it was a very long running source of confusion for me.
This is a bit long and grammary, but I highly recommend it.
What does passive mean in grammar? According to “Grammar and Composition Handbook High School 1” (Glencoe, McGraw-Hill):
passive voice: An action verb is in the passive voice when its action is performed on the subject.
Here, the action is performed on the subject, whereas in Japanese, the action is always performed by the subject.
When is the passive voice used in English? From the same book:
Generally the active voice is stronger, but at times the passive voice is preferable or even necessary. If you don’t want to call attention to the performer of the action or don’t know who the performer is, use the passive voice.
The big takeaway here is “if you … don’t know who the performer is”. In Japanese, the subject is left unspoken all the time, and there’s really no distinguishing between a sentence where the subject is unspoken yet known from context, and a sentence where the subject is spoken and unknown even within context.
Here’s the main portion of the sentence from the manga (with the subject added and the conditional removed):
Passive voice: “Kanami-chan was rejected by her brother again.”
In this sentence, the brother is performing the action of rejecting. However, in Japanese, the が-marked subject is the one performing the action. We know the brother cannot be the subject, because he’s marked with に.
Active voice: “Kanami-chan received rejection from her brother again.”
Here, Kanami is the one performing the action. What is the action? “Receiving rejection.” This is consistent with Kanami being the subject of the sentence, and her brother not being the subject.
This form, which is explained on this page of a Japanese grammar site, is known in Japanese as 受け身.
What is うけみ? If you look it up in a Japanese to English dictionary, it’ll simply say “the passive voice”, but this isn’t an actual translation. This is “there’s no English grammar term for this concept, so here’s an English grammar term for a completely different concept that sounds a bit similar”.
If you break 受け身 apart, you get:
受ける: to receive
That matches up with the active voice sentence I wrote above, where Kanami “received rejection”.
The Japanese grammar site goes on to define/explain うけみ (I’ve added English translations), starting with example sentences:
Example sentence: 「他人 に 笑われる。」
「笑われる」は、話し手（書き手）が他人の「笑う」という 動作を受ける という意味を表しています。
In the word 「笑われる」 (the うけみ form of わらう, to laugh), the speaker (writer) is is receiving the action from another person laughing.
Example sentence: 「観客 から 声援を かけられる。」
同じように、「かけられる」は、観客の「かける」という 動作を受ける という意味を表しています。
Likewise, the word 「かけられる」 (the うけみ form of かける, to address someone), the subject is receiving the action of being addressed by the audience.
どちらの例文も、 【―に】 または 【―から】 の形の文節が「笑う」「かける」という動作をする主体を表しています。
In both cases, the word or phrase marked by 【―に】 or 【―から】 marks the actor of the action “to laugh” or “to address”.
このように、 他からなんらかの動作を受ける ことを 受け身 といいます。
In this way, the concept of receiving an action from another is called うけみ.
Anyone who’s seen CureDolly’s grammar videos on this form know she prefers to call it the “receptive form”, a term I personally agree with, and use myself.