No extra fees or permits needed.

The Three Sisters Islands are inside Niagara Falls State Park, and the park itself has no entrance fee. You just walk in. There’s no separate ticket or permit to step onto those islands.

If there’s a cost at all, it’s parking. The lots near the park charge, and that’s your only expense. So no, you don’t need to buy anything extra to access the islands beyond whatever you pay to park your car (or nothing if you walk in from town).

The islands are literally a short bridge walk from the main path. They’re free. Go enjoy them.