How about making a little photo album, either cutting and sticking or the photobooks from boots. Put together some memories, perhaps of your bro as a little boy and the first time he introduced his fiance, funny or touching moments. Then at the end you could give them a voucher for something they'd enjoy, a weekend away, cookery lessons or a show, tell them to carry on the good times and wonderful memories.
I did something similar for hubs and my anniversary.
Em x