Awwww!! I am so sorry that you and your girls are in this situation. I am sure that the stress is hard on all of you. I have heard that any traumatic or major changes that happen in a child's life can cause this to happen.
I'm sure you are giving her a ton of love and support, but I think you still need to tell her that if she can not make it to the bathroom, she will have to wear diapers. She does need to be motivated and that usually does it, especially at her age. I know that most preschools and daycares do not take them at that age if they are not potty trained. It will become another hardship for you if you can't find a place to take her.
I feel so bad for her...my heart just aches. It is so hard when divorce happens and the children are just put right in the middle of it. They didn't choose it and they definitely don't understand it.
I wish the best for you and your little family!!! Good luck and have a very Merry Christmas! Be grateful that you have each other. It could always be worse. :)
Take Care!