The Big Blue Marble (TV series), "Witch's Sister" (1979)
IMDb synopsis: Lynn Morley, a 10-year-old girl with quite an imagination, believes that her older sister, Judith, is being trained as a witch by their odd neighbor, Mrs. Tuggle. No one else seems to notice Judith's strange new behaviors and habits, like reading Tarot cards and spending time with a mysterious young man whom she apparently raised from the lake near their house. Convinced that she alone can stop her sister from falling completely under Mrs. Tuggle's influence, she and her best friend, Mouse, attempt to gather evidence. Before they can find a way to vanquish their foe, Lynn's mother is called away unexpectedly, and Lynn and her siblings are left in Mrs. Tuggle's care on a dark and stormy night. Will Lynn be able to break the spell before Lynn's little brother becomes Judith's victim?
Here's Part 1 on YouTube: http://www.youtube.com/watch?v=W73dds9U1ZQ