They close in winter and during high water.

Yes.

Three Sisters Islands are part of Niagara Falls State Park, and the park does not mess around with safety. The pedestrian bridge to the islands shuts down when ice makes it slick, which is basically all winter. Same deal during high water in spring or after heavy rain — the river rises, the current gets nasty, and they close the islands.

It’s not a scheduled thing where you can plan around “closed December–March.” It’s more of a play-by-ear situation. Some winters they close early, some they stay open later if it’s mild. High water closures usually happen April–June but can pop up anytime after a storm.

Check the park’s Facebook or website before you head out. If the water’s running fast or snow’s on the ground, assume they’re closed.