Hi, I'd like to help you with your question.
It is very stressful to have to evict someone, let alone someone related to you. No one wants to have to take action in a situation like that. But it happens to a lot of people. So you are not alone.
Keep in mind that while you may feel bad about this, your niece is not doing her part in being responsible. Ultimately, this situation is on her. She did not hold up her end of the bargain, and as a result she put you in a bad position. The problem with people who act irresponsibly is that they like to make you feel bad about what they have done. It's how they handle it to make themselves feel better.
If your niece was being responsible, she would have contacted you by now. She would be letting you know about her situation and keeping you up to date about her plans. She would also be either moved out or have given you a date she would be out. She is not doing these things so you are left with the anxiety of not knowing.
Remind yourself that you are doing everything you can to handle the situation. You have given your niece adequate time to be ready to move out. You have a back up plan on what to do if your niece does not follow through. And keep in mind that if she does not follow through, it is not for lack of effort on your part. You tried. She needs to own up to the responsibility. Don't let her make you responsible because you are not.
I hope this has helped you,