Can you try shopping around a bit? Ask her for the specific shoes that she wants and then try and see if you can get them somewhere else (or something really similar)

Also, do you seriously need your nails done? No-one will ever see your hands anyway, and the same goes for makeup - just do your own.

Hair I can understand because she probably wants everyone to look the same - but again - check with her to see what she is imagining.

The dress is a little bit more problematic - but given that the wedding is still so far away maybe you could suggest that some more thought is put into something a little less expensive.

I don't know of any way of asking a bride to change her vision - the only ones I have come across have all been a bit bridezillaish - but I would speak to your bro about it. Explain that you are really happy to be involved and really excited but given that you are on a budget you are really struggling to come up with the cash and is there any way of making it cheaper. Its his wedding too, you are his sister and as far as I am concerned he should be the mediator.