I have visited Sanibel Island a couple of times and fished it a little bit once. A real easy way to get in a few hours of fishing is to visit the Ding Darling Wildlife Refuge, where they rent canoes for use on their wildlife trail/waterway. The canoe was really cheap ($10/day) and allowed my wife and I to try our hand at snook. I caught one small snook and stalked several (I had my wife paddle/sneek up while I fished).
If your BIL only has a small amount of time, this might work.
The other option is to do the drive through the refuge a couple hours before dusk and fish the brackish water areas around the culverts. He'll be surprised at the amount of activity there.