Thanks, Much Purplexed!!
Normally the partners of the people in the wedding party are seated together at a special table up near the head table (top table). The exception might be if these people have close friends who will also be among the wedding guests, in which case, they may prefer to sit with their own close friends as opposed to with people whom they may not know.
In terms of your fiance’s brother, there is no problem with having him seated at the head table. As long as space permits, it is not uncommon to have special people who are not part of the wedding party seated at the head table (you commonly see this with grand parents, for example).
Normally seating at the head table is arranged as such:
Bride & Groom in center
Maid of Honor to the right of the Bride
Best Man to the left of the Groom
Bridesmaids to the right of the maid of honor.
Male attendants to the left of the best man.
Parents of the bride to the right of the bridesmaids.
Parents of the groom to the left of the male attendants.
Any special people on the bride’s side would be seated to the right of the bride’s parents.
Any special people on the groom’s side would be seated to left of the groom’s parents.
With respect to the bridesmaid who is wheelchair bound, she will, of course, sit at the head table with the rest of the wedding party. Just make sure to pre-arrange to have someone wheel her to her place at the table. She should take her place in the procession to the head table along with the rest of the wedding party, certainly not separately.