You better hope she gets her Hogwarts letter next year and gets the hell out of the house.

You should tell your parents to get her to see a psychiatrist. I was first diagnosed with Aspergers, depression, bipolar disorder, and schizo-affective disorder when my parents read my diary and found out about my plans to hold my english class hostage and murder the classmate who'd been bullying me. She'll be mad at first bat after a couple years treatment she'll thank you.

Please take a screenshot of her diary and post it on Reddit. Please and thank you.

Did she forget to take her meds?

Hopefully she wrote that stuff in there just to mess with your head. When you go snooping, you're likely to read something you don't like.

I feel like OPs sister must have given her a good reason to read the diary in the first place

******* RUN!!!!

This is like a modern day Single White Female!

You should ASAP inform your parents about it and they should take her to the therapist. This might sound like funny thing to people on internet but it's not. She is basically thinking of murdering you in real life...you should keep yourself and your parents safe from her.

Snooper stop snooping. Your sister is going through a phase. Your life is not in danger.