2ndtimecharm, why don't you inform your FH that this is the 2000s and the new big thing is to have mis-matched wedding attendants (ie: uneven numbers of BMs and GMs OR having opposite sex attendants like bridesmen and groomsmaids) and that if he really wants to have HIS sister stand up for HIM you'd totally support him asking her.

and then insist she has to dress like the boys.