The groom can have his sister stand up for him as one of his groomsladies. She can dress like the other bridesmaids and stand on either her side or his side for the ceremony and pictures. Assuming he wants her as one. If not, they have the decision on who is in and who is out.
EDIT: If you are the SIL, unfortunately the decision is up to them, don't take it personal as anything against you. Be happy that you will not have to worry about rehearsals, remembering toasts, buying a new dress, etc, etc. You will be able to enjoy the day as a guest, no responsibilities.