She didn't talk with you about this new relationship she's engaging at all?
Did you guys had a poly arrangement on your own relationship previous to that? If you do, what are the terms?
It sounds really inconsideate of her to just decide on her own she'll be in another relationship and never consider your feelings on this, even if you guys had a previous poly arrangement.
I do realise it IS a stress factor, but nevermind her mother for now. Did you comunicate to her how you feel about it? How do you feel about it? Is your problem with her sleeping with him, or sleeping with other people in general?
I'm sorry you're going through this.