I am so sorry for your loss I can't even imagine your pain. I am sure celebrating right now is the last thing you want to do but would your sister want you to cancel? Do what is right for you, don't worry about the guest, the $$$ or anything else not even your FI worry about you. You are allowed to be selfish, to only think of what is best for you, and your feelings. You and your family are in my prayers and remember we are all here for you and to listen