My sister found my journal?

Well, even law agrees that it is allowed to break confidenciality if there is a risk of harm for you or others. You are planning to harm yourself and lost all will to care and love yourself. If she tells your parents, as she should, they will take you to a mental health practitioner and you will stop feeling that way, even if it takes you long talks out of it or meds, or whatever. Dont get mad at her, it would not be considered a gossip if she tells away, but a cry for a solution.

Talk with her before she moves it to your parents.Soemtimes taking can help to find to yourself. Maybe you need psychological help, because it's not normal to think always about suicide.

If you are actually making viable plans to end your life then you need professional help.
Since you are only worried about Mom and Pop finding out then I do not believe that you are seriously considering this at all and hopefully when told they will see this. At worse you will be referred to a mental health specialist who will confirm this

You should find a way to correct those thoughts, change the way you do things, environment, or even professional help if nothing else works. Life is short enough as it is, don't make it shorter and lose out on some great events that may occur. Or focus that energy into actual books and become a professional writer.

If you close your eyes, you can very well think that the whole world has become dark. It will be night and all people will go to sleep. You too can imagine some such solution for your self-created problem.

Here instead of writing something about your sister's present mental healthy, you should not try to write stories.

After all it is a sentence for which no answer can be found anywhere.