Big Brother is the companion site to Little Sister on the corners of the Third and Fourth Sister reefs off Tongue Reef. Wavelength Reef Cruises describes both corners together: very good coral cover from large boulder corals to patches of branching corals and huge clams, with numerous reef fish and larger pelagic species, especially very large barracuda keeping station over the deeper boulder corals. Corner sites benefit from water exchange as the tide sweeps past, and Big Brother is no exception.

The flow keeps the water clear and feeds the corals and fish. Boulder corals form massive mounds, branching corals form thickets, and giant clams nestle among them. The fish life includes schooling fusiliers, parrotfish, wrasse and butterflyfish, with pelagic visitors such as barracuda, trevally and reef sharks.

Wavelength does not publish separate details for Big Brother beyond the shared description, so the two sites should be expected to be similar in character. No depth figures are published; the snorkel area is shallow. Like the other Sister reef moorings it lies on the seaward side of Tongue Reef, about 90 minutes from Port Douglas, in a low-impact area with exclusive operator access, so it is never crowded.

Wavelength is a snorkel-only operator whose marine biologists guide in-water tours and give reef talks; its skipper chooses three sites each day from the company's exclusive moorings according to wind and tide, so a particular site is never guaranteed. Scuba divers wanting to dive the Sister reefs need a private charter, and because the site names are Wavelength's own, other operators may not recognise them.

Wavelength Reef Cruises departs Crystalbrook Superyacht Marina, Wharf Street, Port Douglas, at 8:00 am and returns about 4:30 pm; park at the marina or on surrounding streets. The Sister reefs lie just off the seaward side of Tongue Reef and the crossing takes about 90 minutes, threading through the Tongue Reef maze. Wavelength is snorkel-only and holds exclusive moorings here; snorkellers enter from the stern platform at the mooring.

Scuba access requires a private charter with a small-group dive operator.

The Sister reefs sit on the seaward side of Tongue Reef close to the shelf edge and are exposed to the south-east trades that blow from about April to September, so moorings are chosen to suit the wind and large swells rule out the exposed sites; a 25-knot-plus trade wind makes the crossing rough and operators may cancel. Exposed sites on the seaward face and in the channels through the major reef gaps are dived only in light winds and suitable tides. October–December is typically calmest with the best visibility; the wet season (December–April) brings northerlies, storms, cyclone risk and marine stingers (November–May).

Tidal currents run through the gaps between the Opal reef sections and around bommies, strongest on spring tides – channel and bommie sites are timed for neaps or slack water.

Snorkel from the boat along the corner with the guide, staying in the lee if current runs, and look out over the drop for barracuda and other pelagics.

Wavelength Reef Cruises – snorkel-only full-day trips with exclusive (low-impact) moorings at Third and Fourth Sister; scuba only by private charter (Scuba IQ / Blue Dive, Tech Dive Academy).