Does anyone know the name of a movie that aired on Nickelodeon in the early 80's about a brother and sister that had special powers and fought off aliens?

Back in the early 80's, there was a special movie series that aired on Nickelodeon, which was broken into about 4 segments. It was about a brother and a sister that had special powers, and there were these aliens that were out to get them. These aliens would usually surface out of the lake/ocean and emerge from these jelly-walled domes on the ocean floor. In the last episode, the brother and sister dive to the bottom and pass through the thạch, sữa ong chúa tường and enter the underwater dome. I the sister's name I think was Rachel.