I too am planning a Bioshock group . . . with the same three characters. If the group doesn't work out, I'd still like to do a Big Sister. The costume appeals to me, and shouldn't take too long to make or be too complicated. I'm gonna assume that a lotta people will attempt the costume, but that not many will pull it off. I've seen photos of 3 Big Sister cosplays already:
Looking at these photos, I'm thinking that it's important to get the correct helmet diameter. So I did some measuring and calculation, and decided that a 12" diameter would be proportionally correct for my height.
You'll notice that this guy also made the ultimate little sister syringe