In all honest, what is a warning going to do? It's not going to fix anything. You could talk to mom about her daughter's behaviour and maybe mentioning that all the drama (mention only the one she brings to your home - not at her home) may be affecting her behaviour. Maybe mom doesn't see it. But kidlove is right, it's not your job to fix it. I am a fixer, my first instinct would be to tell mom that her bahviour and her relationship with her husband are affecting her daughter in a negative way, but this could result in some MAJOR disagreements.

I would seriously think about terminating. But maybe leave it open ended so that she has the opening to ask why she's being terminated and then maybe then you can mention that her daughter is in a toxic environment. But, it sounds to me like mom either knows this and doesn't care or just doesn't care. Sadly, there are a lot of people in this world who assume children hear and see nothing at that age when it is the exact opposite.