i wouldn't be disgusted. of course the person would be flattered to be asked, but some people don't like that sort of attention. i was BM for a friend before, i was delighted to be asked, but afterwards i swore i would never ever do it again after having to put up with so much aggro e.g. her mother's comments and having to wear a dress which did not suit me at all. however, another friend asked me to be a BM this year, and i said yes straight away - i think it's very flattering that someone would think that highly of you, and it is appreciated. it was very nice of your FSIL to think of you. if you're not keen, don't bother. you're being honest and you will still be there on the day etc.