At the end of the day, it's an e-mail address and you are not limited as to how many hotmail accounts you have. If you can't get it back for her, make a new account. Make sure you make a _strong_ password and when you choose a secret question/answer have something like.
Q. What's my favorite food?
A. I like to pick my nose
It's gross but effective.
And should this happen again somehow, your sister will tell them her favorite food and it will be wrong!
In any case, if this does happen again and they succeed. Be sure to save her contact list on a regular basis, so you can import on another address.
Contacts > Save Contact List...
Contacts > Import Contacts from a File...
Best of luck.
-Andy