A lot depends on the type of wedding she is having. You can have them involved in various ways depending the wedding plan itself...
Make them each a hostess - handing out the programs at the church, greeting guests at the reception and directing them to: their seats, the guest book, the gift table, appetizers, drinks/bar/restrooms, etc. They can also serve the cake after it has been cut.
Have them do some of the readings for the ceremony, or recite a poem of her choosing.
She could ask them to gather pictures/memories of their brother through the years to create a slide show. Often the couple will have a slide show playing during the meal with pictures of them growing up and as individuals, and then closing with photos of them together over the years.
They could be in charge of creating a gift basket and selecting the music for an anniversary dance. Either in lieu of, or in addition to, the boquet/garter toss, it is nice to have a song played where you invite all married couples on to the dance floor for an anniversary dance. The song will play and the DJ (or whomever) will ask couples married 5 years or less to sit down. It goes on until only 1 couple remains who have been married the longest. The get the gift basket and the opportunity to give the couple a bit of advice if they'd like.
She could ask them to together, or individually if easier, write a toast to their brother for the reception.
Lots of ways they can be included, just get creative! Good Luck!