Wow that has got to be difficult for you and the poor child. I would talk to her mom without the child around for starters. I would also tell her that in your opinion the comments her caregivers say can not reinforce how she is behaving as okay. Talk about you all doing the same things for example, ignoring her, giving her some firm reassurance she is okay and then nothing more about it, giving her a safe place to go when she is upset so she is allowed to feel sad but then she soothes herself and then comes back to play. I think you are correct that the mothers responses are part of the problem. I think she will actually improve when she is in a larger setting in school even if you don't get a handle on this behaviour while she is still with you.