I'm actually looking for the same thing you are for your sister. (I am a C-cup) I'm using a compression bra that I got at Target for 15$ but you know, it's still a little revealing. (Pretty much looks like my character has man boobs, not breasts) I am considering doing ace bandage over it but I've heard that can be really bad for you (especially since i'll be walking around the con for four days, about 12+ hours a day.) So I may lurk around this thread, as i'm curious to the same things you are c:
Also, Compression shirts are certainly helpful but they can be very very costly...