There's never a great deal of availability in RCI for the resorts on Sanibel. Since you have a small window you're looking for, even though it is in hurricane season, you'll be lucky if you find something and very lucky if you find two options.
We've stayed at Caribe Beach Resort, Casa Ybel, Lighthouse Resort, Sanibel Beach Club, Shell Island. We'd happily stay at any of the resorts on Sanibel. The resort you're most likely to find something at is the Caribe. It's mostly studios, nothing fancy and an older resort. We've stayed there at least five times and enjoy it. We have a Christmas Day to New Year's Day reservation at the Caribe and are looking forward to going back.
We really like Captiva too. However it takes longer than you think to get there from Cape Coral. We have friends in Cape Coral.
In case you don't know the bridge is the only way on and off the island and there is a $6 toll to get on the island if you have a transponder. It's $9 if you don't have a transponder. Getting off the island is free.
There are nice resorts on Fort Myers Beach. You might want to watch for something there in addition to watching for something on Sanibel.
Naples is a little farther but not that far and we really like Naples too. If you can find something at the Vanderbilt Beach Club or Gulf Pointe Intervals I can recommend both.