Nancy, are your brother and his GF flying in the day of the cruise or a day before?
Carnival offers a transfer from both IAH and Hobby the day of the cruise. I think it's about $35-$40 pp.
If they're coming in the day before the cruise, I'd suggest they rent a car and drive to Galveston.
We've flown into Hobby the day before, stayed in an airport hotel (with free shuttle) for the night, then gone back to Hobby and taken the cruise line transfers to the ship. It worked well