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?

How do you talk to her about it? Just do it. She is your wife as much as she is a mother to her children. If it bugs you so much, tell her that you will feel comfortable sleeping with her in the same bed at night and see if you can make arrangements. Maybe her children can sleep in the same room but on mattresses or something.