Yes, that's exactly what PP are saying 
If a man with existing DC goes on to remarry, and have more DC, HE needs to think of the implications. There's too much of this "but you knew he had children...." as some weird justification as to why a mother's own DC should go without, in order to placate children that she has no biological relation too, because their bio mother and father can't/won't provide the same.
If the existing children are going without, then their own mother and father need to sort it out. Not at the expense of children who have a different mother. That's for their mother and father to sort out respectively.
OP - re your holiday, I would ignore all the "oh, but those poor children" posts who are missing the point that you are planning a Disney holiday for primary aged children, and the SC are going to be in 6th form. They are going to be bored stiff for a lot of the time, or you and DH will have to separate so one can do the younger activities and one the older. We have 3DC, toddler twins and teen DS. Teen DS has been away with school, his grandparents, several times in the last year and had a blast. We've taken DTwins, once, to a tots style break, while DS stayed with grandparents because he would have been bored beyond belief. That's just common sense. At no point was anyone pearl clutching at poor DS, denied a holiday by his horrible parents.