I was in a similar situation. My twin sis was getting married close to my due date. I declined her invitation to be a BM. I do feel i missed out and am guilty at not been there for her but in my case i did end up having an emergency CS and i went 12 days past my EDD so the birth was even closer to the wedding than i thought. Personally i dont think i could have done it and if i had said yes i think i would have ended up letting her down which would have been worse than saying no in the first place.
Hoody