Her thoughts are probably the same as mine ENOUGH!
Enough of the adolescent behavior. This is the problem with so many message boards. It becomes like an adolescent clique where certain people drag up every old post, like a 13 year old would drag up every social faux pas committed from kindergarten on, and use it as an excuse to trash the OP. I honestly thought that this board was different, but maybe, I am beginning to see that my expectations are too high. Really, if you don't have anything helpful or informative to add, or if you, for some reason, really have an issue with the OP, then skip her posts. The constant picking is childish and unnecessary.
OP, honestly, if space is for the "stuff" is an issue in your home, then maybe politely say to your mother and sister that this is the issue, and ask, as a PP said, that if they really want to spend their money then do it on things that will create memories, but not take up that much space. Like a photo album or scrapbook. On the other hand, you can always donate/Ebay/Craigslist the items that you don't have space for or your daughter outgrows. I think the costumes, due to their expense, usually sell well.
On the idea of "keep it at your house:" this bothers me when people do this. It as if they are saying, basically, "I don't want it." Which to me, is rude given that someone spent the time, money, and effort to pick out something they thought would bring your child joy. I have been taught to always accept gifts graciously and thankfully, whether, or not, you like it or want it.
I brought up the past post because the OP wanted help to say no. It was a suggestion for her mother and sister to spend the money to bring her stepchildren. Then the OP would not have to worry about her husband feeling like he had to go because he didn't really want to go to WDW.
Another problem with boards is when someone reads something into a post that isn't meant and acts as the post police.