I can't help but smile when my stepsister brings home her book report filled with red marks.
That's not something for you to smile about. Did you do something for her to receive red marks?
She studies at yale and she submitted this homework but I edited it before her submitting it.
You are not being a good step sibling when you have sabotaged her homework. You are supposed to be there and support your sister when they are failing. What you did is made her life bad with this sabotage, you need to stop messing with her education and grades and help her recover from this.
It was my first time, I was just kidding. Sorry, I'll stop.
I can see that you're being sincere, and I hope you don't sabotage your stepsister again, as you may not like your living arrangement with your stepfamily, but it is not her fault, and you're unnecessarily making her life difficult. So apart from you apologizing to her, I hope you also strive to help her if she needs any help from you.