Jared, When I was in Naples a couple of years ago I had some luck very early in the mornings on the town beaches. Once the bathers show up things pretty much shut off, but I was able to get in a couple of hours of sight fishing for jacks and misc other fish. Fly choice didn't seem to matter much as long as it was white. While I was there I saw a med sized tarpon(~75lbs?) caught by one of the bait fishermen off the naples town pier, so they are out there...
Have your brother in law get in touch with The Everglades Angler in Naples, they were very helpful for me and if he's considering a guide they work with a number of good guides as well.
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.
Although wading can lead to some really good fishing on Sanibel, a boat is the way to go. There are many spots that are accessible within 15 minutes of the island by boat. If you are into Red's...then check out Redfish Pass...if you are into Tarpon...then check the beaches or the deep passes...or better yet check the Ding Darling Refuge. The Baby Tarpon and Snook will begin to move into this area during this time of year. There is a large amount of food for them and it is much warmer than the outside beaches. If you need a guide, I can give you a name and number of one the best guides in florida.