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?

Ofcourse it is okay. You got to realise that she is the mother of those children, and now you too are their father. If you will not accept the kids as yours then it will be difficult for kids as well to accept you as their father. Also, kids are very sensitive and when it is difficult for elders to accept little kids as their own from other marriage then just imagine how difficult it will be for kids to accept a new person as their father. Your wife married you because she wanted to live with you, so do not doubt her intentions. She must be sleeping with the kids so that they do not become insecure and because of which no more troubles arise in the relationship between and her and so that the kids do not start disliking you. I think she is trying to settle them, so that she can comfortably spend time with you without being bothered bout the kids feeling insecure. But if this thing is bothering you much, you cantle to her. Tell her that till the time kids get settled with you being around, two of you can have your own time when they are in school or have gone out playing. Or off and on you cantata a break from office and have date with your wife when the kids have gone to school.