I can commiserate with you on your frustration with this mother, who clearly leaves a lot to be desired as a parent. But I'm pretty sure you got the answer clearly: You must follow the court order. On top of that I can see that at least some part of your conflict seems to be your fault. Please understand that you are hurting the kids indirectly when you go tit for tat.
I would caution against opening the Pandora's Box of child choice. I hear you saying that the boy doesn't want to visit so he should get what he wants. You add that you also think it would be right not to send the sister if she decides it too. That may sound like a good approach to you at this point in time.
It is a horrible mistake. For one thing, the kids won't always be at an age where they recoil at their mother's bad language and habits. Have you ever raised teenagers? They prefer the parent who rarely checks on them and would probably look the other way over drug use or underage drinking, smoking or sex. If you tell them they can't go to that party they would DIE if they miss, they will easily be convinced never to see you again.
Don't make the mistake of saying that would never happen with your stepkids. I'm sure they aren't like that now, but you already have a stepson who is ready to not see a parent. Just wait until he starts hanging with the wrong crowd and you don't approve. Think you have the right to tell him who his friends can be? You need all the shakey parental authority you can muster to get through those years and you are a fool to give it away while they are at this age.
Kids also can be put under intense pressure and guilt when put in the middle between two homes. You don't see it now, but the changes will come. If you have set up a situation where parenting is in their hands - watch out. Reeling that monster back in is a lot tougher than never discussing it in the first place.
If you are ok with sending the girl, then you see no danger. Send the boy. There are a lot of things kids can decide and a lot of things they can't. This is a can't.