If you are having problems with the stepchildren, discuss these problems with your spouse privately, not within earshot of the children. Keep in mind that children are nosey and will listen through the walls. Regardless of your feelings about your biological counter-part, don't make derogatory comments about the other parent to the children. This is alienating and damaging to the child and your relationship with the child. The child will usually want to defend the targeted parent but will avoid saying anything because that's easier than confronting you.
Children live by a simple principle, went uncomfortable, avoid.
Don't be fooled by their silence. You will probably not hear their thoughts. They will have opinions and feelings about what you say. You just won't know what they are thinking. Your stepchildren still need time alone with their biological parent.