What do you think of reading my sister's diary?
What do you hope to accomplish?
I want to know more about her and her life.
That is wrong to read her diary. That is your sister's private life. You have no part in it.
I read her diary when she was younger and I'm not going to stop now.
You will be in trouble when she finds out that you read her diaries. Might as well stop now before that happens and destroy your relationship with your sister.