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 halaya pader and enter the underwater dome. I the sister's name I think was Rachel.