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?

You must not forget that apart from being your wife she is also mother to her children. Kids are very sensitive and if they be neglected there will crop up endless problems in your life. Being an adult you can understand reasoning why your wife is sleeping with your kids, but the kids might not understand why their mother is no more sleeping with them like before. And they may start hating you, which I am sure you wouldn't like. SO, I suggest give your wife a little time to set things fine with the kids and you can have a chat over this with your wife and see what she has to say.