Wow, I really can feel for what you're going through right now. My FI's sister (also as my bridesmaid) passed away in July and we went back and forth on what to do also. Ours ended up working out due to one of his guys backing out, but you must decide what to do. Not many people understand what's going on with your FI right now and really can't relate. What we're planning on doing is releasing butterflies at our ceremony to represent his sister in memory.
I'm sorry for your loss - that's a very, very tough situation. It'll all work out!