I should have read the whole thread before replying.
My opinion still stands regarding paying 1/2 for extra things and the issues with travel, however, it seems to me that you are not really looking for opinions, but looking for people to agree with you.
In all reality, when you married your husband you knew he and his son are a package, regardless of any more children you choose to have. If you can't afford to take everyone, then you can't afford to go. You husband's ex does not have any obligation to your daughter, as there is no relationship there, so what she chooses to do when your daughter is not present is a moot point. However, there is most definitely a relationship between your husband and his son, so there is an obligation to include him in the family, because he is a part of your family. You are accusing the ex's family of playing tit for tat, but you are doing exactly that yourself. "My step son gets to got to X,Y, and Z, and our daughter doesn't, so it's only fair. No, really, it's not. Again, your daughter is not part of that family, your step son is very much a part of yours. So, yes, when it comes to big trips like WDW, it is NOT right that you took one child and not another.
In regards to child support. The court does not care if you decide to have 1 or 10 more childre. Their stance on it is, "You knew you had child X when you decided to have more children, if you can't afford to support them all, you shouldn't have had more." (that goes for both your husband's ex, and you)
I will give you background. My ex had a daughter from a previous relationship. Her mother remarried and had another child. She was rarely included in anything that her mother did with her new brother. The excuse was always, "you were with your dad." She always felt left out and like her mother cared more for her brother than her. It didn't matter that she got to go to WDW and on other vacations with us (not that she didn't enjoy them and wasn't grateful) but what she did see was that he mother didn't want to do things with her that she did with her brother. In a child's mind that translates to the other child was more loved.