Just one observation, K.... if you are shouldering the emotional burden of your sibs not getting along, your parents don't really have to address it with them. You are getting mad FOR them and then they get to be in the 'helpless to change it' role.
Do they really feel that way? And are you voicing your disgust at their asking to stay with family/get rides to your folks or sibs? I would keep a lid on that.
As someone who has seen a lot of convoluted family dynamics, I've learned to make my own boundaries that I can live with, and let others do the same. I won't do it for them. I have watched one sibling rely on my folks to the detriment of my folks, and while other sibs got frustrated with that sib, I just reminded myself that while it was appalling, it was *their* stuff, not mine. Your parents are capable of telling your brother "oh, no, we won't be able to get the party stuff"... they choose not to. That's *their* stuff. Don't take it on. There's nothing progressive that will come from it. Just know in your own heart that you are doing for your parents as YOU feel you should and let that be your guide.
(I just have to add: are your parents asking the sisters what they need before they come, or are the sisters demanding this with their shopping lists? My in-laws were very sweet and did ask us what we needed when we visited; we paid them back for the groceries and then made sure that we did a shopping trip with them, picking up our own baby items and food for all of us for the next several meals. So they were different scenarios....)