[quote= Thing is - I've already spent close to $800 to gussy up as her bridesmaid and can't afford to give them as much as I'd like to.
How should I avoid feeling embarrassed when the day rolls around?[/QUOTE]
By giving what you can afford with a big smile and a hug.
Perhaps by making a personalized basket--say if they are going to the beach on their honeymoon, you get a beach bag, two towels that you have monogrammed, some magazines, sunblock and sun visors. You can do that for about $75.
Or have room service deliver champagne or flowers to their suite welcoming them on their honeymoon.
But to be honest, only you control how you feel. If you do the best you can afford, then there is nothing for you to be ashamed of. (It is your sister who should be ashamed for her overwhelming lak of good taste and etiquette)