I accidentally read my sister's diary

I know this is wrong, but I couldn't help it cause her alarm so annoying. So I was looking for alarm settings in her phone and what I got is her diary. And it just one sentence 'I hate myself' and she barely turn 13 next month. I feel so horrible right now consider me and her are very close. I know she has this tendency cause she drop out at 3th grade and she barely has no friend. I can't tell her diary to my parents cause you know, parents. Is my sister just in her phase right now? Is she need help? But I will turn really annoying if 'out of nowhere' I ask about her personal feeling cause she really hate that stuff... What should I do? Act like nothing happens?