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?

Honestly I see no danger sign in this. You need to recognize and respect her mother instinct even if the kids aren't yours. Her kids need her as much as you do. However if it bothers you that much, put across your point of view... Hey why don't you sleep with her and her kids in the same room? She wouldn't mind that. In fact she would love and respect you more. Slowly but surely your issue would be resolved!