Well she was determined.
You need to stay strong. For yourself and for your younger children..
You need to make sure your younger children stay on their schedules.
If they ask about sister, just tell them she is will be gone for a little while.
If they insist on contacting her, have them draw pictures. The 5 year old could write letters. Date them and save them for when you do have contact with her again.
At 18, honestly she is seen as an adult by the law.
It will be interesting to see if she really is going to be able to handle all of the responsibilities that she will face.
As a mom I also would be heartbroken and worried. But I also would need to stay strong and at any chance I might have, make sure she knew I would always love her and she would always be welcomed home.
She made this choice. She knew how you felt. Now she will get to take the responsibilities for her choices.
The whole point to parenting is to raise independent adults. Sounds like she was ready.
Hang in there. I am sending you peace and strength.
I hope that she makes good choices and decisions and is strong enough to admit, if she needs to come home and ask for your help.
I hope that if/when she comes back you will be willing to start over with her. To listen to what it is she feels she needs and then that she will listen to you and what your expectations and the 2 of you can work this out.