Thank you
Earlier worries about spending money for pool trips and ice cream don't seem to jive with this concern. I would be terribly worried about what you are showing the other members in your immediate family by what you are about to do.
When you married your DH you knew he had kids. Your family knew you were getting kids in the deal. You became a family when you said, "I do". Taking one and not the others doesn't seem right. Worrying that 'they' won't have money to even go while your DD will be given everything seems like a lack of concern from my POV.
I hate that is seems like blood-relations are closer than others-it can be that way BUT, your DD is so young and you were/are so worried about paying for
the next trip I don't know why you haven't explained to your family that waiting until all the kids could go would be the best for your ENTIRE family. You are now the leader of a blended family and that take extra special talent.
I think it would be great for you, your mom(their grandma too, in a way),their aunt and all the kids have a terrific holiday that they can share. Shared memories for kids is a wonderful family bonding opportunity. I really hate the yours, mine, ours set dynamic that seems to be happening.
JMHO-YMMV