I used to own a car.
A couple of years ago, I wasn't using it much (I live in a big city and walk to work), so I lent it to a "friend" who needed to get around. One day, I got a nastly letter from a small town where I had never been, saying that I had an unpaid parking ticket - now very late, incurring extra fines. I had actually been in England on vacation at the time of the citation. Since my "friend" hadn't paid, I was "prima facie" (on the face of it) responsible for the ticket, since it was my car.
You are "prima facie" responsible for what goes on with your VMK account.
If you're lucky, you won't get banned for 10 years (I didn't see what she wrote). If/when you get back in, always be sure to either log out of VMK or otherwise lock the PC when you step away. If you're using Windows, Windows + L might lock it - hopefully, she doesn't know your Windows password!
When you get a bank account (or you might already), you sure wouldn't want your sister accessing it and transferring money to herself or her friends. If she did, you could still be responsible because those are probably the rules of the bank's website - since she logged on to YOUR PC, with YOUR password.
What if your sister sent an inappropriate email to a boy she knows you like? Or your teacher?
What if she changed just a few words of a paper you're writing for school?
Locking the PC, or logging out of certain sites is important in a lot of situations - even if you're not away for very long. If nothing else, I hope this serves as a warning to other VMKers to carefully guard your login.
Good luck! Sorry, I don't mean to lecture - I actually hope you don't get banned for too long!

Steve