From a legal standpoint it really depends on where you live. You'll have to check the laws in your State to find out how long a person, that has given no notice of leaving, has to be gone before you can consider her moved out.
You stated that she only left 2 or 3 things in her room. Are all her clothes gone? All her personal items? If so, then I think that's a pretty good determination that she doesn't plan to come back. But just to make sure you can legally take her name off the mailbox, check your local renters laws.
Good luck. I don't envy your position. It's never easy to try to evict someone, especially when that person is family.