It would be helpful to see the kanji for these sentences but here's my attempt. Hope English is OK? Not sure I can explain in Japanese! - Mostly, we were good friends. But...sometimes I got sick of having a little sister. Particularly when it was just us, like when we were home alone together. - Before going to bed, when I was talking (to her), (she) listened (to me) quietly. (Pronouns in brackets because I'm just guessing from the context. They are only implied in the Japanese.) - Sometimes, I wondered if we should just stop sharing stuff altogether. - Knowing what my little sister had done, I became utterly enraged. Maybe I should just give her a beating right here and now, I thought.

I was searching for this particular phrase "いもうとったらいやになっちゃうんだ" and came across this page. This passage is from a the Japanese translation of the children's book "Let's be Friends Again". Both the English and Japanese pdfs are available online for free so you can check it. For me, I am trying to learn some Japanese, but I'm struggling to break down this sentence "いもうとったらいやになっちゃうんだ". That is I can't tell where some words start and where they end, haha. Will be nice if someone could break them up for me