Natural: 동생은 매일 학원에 다녀요.
Assuming by 수업 you mean she goes somewhere to get lessons.

This reads more like a list of sentences than a diary entry. As a list of sentences, it's very good and there aren't any grammatical errors. *As a list of sentences*, there isn't anything unnatural either. (except maybe 수업에 다니다)

However, for a diary entry, you'd actually go through stuff that happened in your day and tell a story. Here, you just have a list of events with no real links between them. If that's what you were going for, great. If not, that's something to keep in mind.

A diary entry might look something like:
오늘은 평범한 하루였다. 엄마는 시장에 장을 보러 다녀오셨고, 동생은 평소처럼 학원에 갔다 왔다. 오후에는 집에서 XXX 축하 파티를 했는데, 언니도 왔고 나도 집에 갔다. 파티가 끝난 뒤에는 나는 다시 서울로 올라왔고, 친구도 공항에 도착했다.

책상 옆에 책장이 있어요 or 고양이는 집안에 없어요 is a pretty random thing to put in your diary :3

Oh also. 일기 means journal as in diary. So maybe you just meant "Friday's Journal" as in "Friday's LangCorrect Post"; if so, feel free to ignore this comment but name your journal something like "금요일의 일지" next time.

Anyways. Good work, keep it up!

감사합니다! 너무 도움이 돼요! :)