How should I handle this situation?

My husband’s aunt is throwing her daughter (his cousin) a quinceanera this Saturday. And at first his said his daughters (my stepdaughters) weren’t coming because of their mom. I asked in advance so I could put in my budget if I have to help with buying dresses and shoes. Well now he says they’re coming this Friday and said what!! What are they going to wear? And is bm doing their hair (braids) as I’m not that good in doing hair . And he says they can wear sweaters and jeans! I said wtf! No! This is a formal event and that’s embarrassing as hell if they show up with us in that. Me and our daughter (11 months) have our dresses and shoes and other things pertaining to the event . My husband even has his outfit . We are all coordinating with color and wearing burgundy and white . So I’m a little pissed that him nor his bm have nothing in order. I help out when I have it but it’s not responsibility either . How would y’all handle this situation? Cause I’m stressed and don’t wanna have them with me looking any kind of way but I also don’t have the financial means to buy everything which is why I asked 3 weeks im advance so I can plan my money accordingly.