Would I be violating my sisters boundaries if I read her secret diary?
Of course, it's a 'secret' diary for a reason. Why would you want to anyways?
It was lying around and the temptation was too strong.
Resist the temptation. Your sisters inner thoughts are a private thing and you shouldn't be meddling.
It's a journal, what's the big deal about it? I'm going to read it when she's not in the room tomorrow. She won't notice!
The big deal is that it's private. If your sister wanted you to know something, she'd tell you, so don't go snooping through her diary.