It is hot, but not Southwest desert hot, but mostly humid hot, which is very uncomfortable. On the water it's still quite nice, but away from the breeze. Summer sees afternoon thunderstorms all over FL, often called pop-up storms. They can be quite fierce, but then they're gone. Lightening is always a risk in FL, so do pay attention and get out of the water.
Sanibel's season is mid-Feb to Mid-April. Orlando's season is pegged to school year, so it's high season is summer and Christmas-New Year's, mid-winter break (usually third wk of Feb), Spring Break, Fall Break (big with folks from UK) and Thanksgiving. The lowest season is early December. It has nothing to do with weather quality, which is even hotter and muggier than Sanibel in Summer because it's inland. Yes, they get the afternoon storms too.
Sanibel does get hit by the occasional tropical storm or Hurricane. The last direct hit was in 2004. Before that is was back in the 1920's, so no, it isn't a frequent thing. Like all sub-tropical/tropical areas, it gets brushed by low pressure systems, some tropical depressions some tropical storms, but that's true of anywhere in FL, along the Gulf coast, the Caribbean, Mexico and Central America. It comes with the location. Peak hurricane season is Sept. Since your stay is so short, I'm not sure insurance needed unless you insure the whole trip for hurricanes. Many policies don't pay for under 3 days, so read the fine print before buying.
Mostly, just relax. major storms have lots of warning time, so you'll be able to rearrange things as needed.