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?

I wont say if its okay or not. No right or wrong about it. How often are the kids with her, and how old are they? If its a weekend affair, let it go. But if its a daily affair, then something needs to be changed. Be tactful during your conversation with her, let her know how you feel, perhaps even expressing how much you miss sleeping with her. Many times, we mean well, but our choice of words play a cruicial part.