Is it okay for my wife to sleep with her children from previous marriage and leave me alone? How do I talk to her about it?

Hi, I don't think there is anything entirely wrong in this, unless and until she is completely neglecting you and it's affecting your relationship. It also depends on how old the children are and how much they need their mother. Besides, it might be difficult for kids to come to terms with their mother's second marriage and so they might need comforting. If you feel that this is negatively affecting your relationship, you must openly talk to your wife about it. There is a good chance she will understand your point of view and find a way out.